Search

Shallowly dipping (<30°) low‐angle normal faults (LANFs) have been documented globally; however, examples of active LANFs in continental settings are limited. The western margin of the Panamint Range in eastern California is defined by a LANF that dips west beneath Panamint Valley and has evidence of Quaternary motion. In addition, high‐angle dextral‐oblique normal faults displace middle to late Quaternary alluvial fans near the range front. To image shallow (<1 km depth), crosscutting relationships between the low‐ and high‐angle faults along the range front, we acquired two high‐resolution P wave seismic reflection profiles. The northern, 4.6‐km‐long profile crosses the 2‐km‐wide Wildrose graben and the southern, 0.8‐km‐long profile extends onto the Panamint Valley playa, ~7.5 km S of Ballarat, CA. The profile across the Wildrose graben reveals a robust, low‐angle reflector interpreted to represent the LANF separating Plio‐Pleistocene alluvial fanglomerate and Proterozoic metasedimentary deposits. High‐angle faults interpreted in the seismic profile correspond to fault scarps on Quaternary alluvial fan surfaces. Interpretation of the reflection data suggests that the high‐angle faults vertically displace the LANF up to 80 m within the Wildrose graben. Similarly, the profile south of Ballarat reveals a low‐angle reflector, which appears both rotated and displaced up to 260 m by high‐angle faults. These results suggest that near the Panamint range front, the high‐angle faults are the dominant active structures. We conclude that at least at shallow (<1 km) depths, the LANF we imaged is not active today. Key Points: Seismic reflection imaging used to explore crosscutting relationships between low‐ and high‐angle faults, Panamint Valley, CAA low‐angle normal fault appears faulted and rotated by higher‐angle structures associated with Panamint Valley fault zoneResults interpreted to indicate that at shallow depths, the low‐angle normal fault is probably no longer active [ABSTRACT FROM AUTHOR]

Highlights • Metasedimentary protoliths in the French Massif Central are Ediacaran. • Eroded rocks were of Neoproterozoic, Paleoproterozoic and Archean age. • Detritus originated from the erosion of the Cadomian magmatic arc and East Africa. • Metasediments encompass 60–75% <100 Ma-old Neoproterozoic crustal materials. Abstract Combining U–Pb and Lu-Hf isotopic data of detrital zircon grains has proven a powerful tool to unravel the provenance of sediments and address continental crust evolution. In this study, we explore the origin of thick siliciclastic metasedimentary units from the high-grade internal domains of the Variscan belt of Europe and examine their significance for Neoproterozoic crust formation and evolution along the North Gondwana margin. We present data and U–Pb/Lu–Hf systematics of detrital zircons from five amphibolite-facies metasediments sampled in the eastern French Massif Central, measured in situ by LA–(MC)–ICP–MS. The sedimentary protoliths were deposited in the Ediacaran as evidenced by field relationships and maximum depositional ages ranging between 592.4 ± 5.5 and 556.8 ± 5.1 Ma. All samples contain three main zircon populations in terms of age distribution and Hf isotopes: (i) abundant 0.55–0.65 Ga zircons with considerably scattered εHf (t) from −19 to +14; (ii) varied amounts of 0.65–1.0 Ga zircons showing dominantly positive εHf (t) with the exception of the c. 1.0 Ga zircons; and (iii) ≥1.8 Ga zircons (mostly between 1.9–2.1 and 2.5–2.8, up to 3.2 Ga) with εHf (t) ranging between +5 and −7. Multidimensional scaling of our U–Pb dataset and a data compilation of Ediacaran to Lower Cambrian (meta)sediments from the North Gondwana margin reveals that the relative proportions of the three age components carry discriminant provenance information. Geological data indicate that the Ediacaran basins of the eastern French Massif Central collected the erosion products of two main source regions: (i) the Neoproterozoic Cadomian magmatic arc; and (ii) the cratonic hinterland, mostly from the Saharan Metacraton and the Arabian-Nubian Shield. Our dataset attests to the reworking of: (i) juvenile Neoproterozoic crust and, (ii) old Paleoproterozoic to Neoarchean crustal components, either as part of the detritus or incorporated in Neoproterozoic arc magmas. First-order estimates derived from the isotopic signature of S-type granitic magmas sourced in the Ediacaran metasediments suggest that 60–75% of the detritus would correspond to young Neoproterozoic crust and thus represent net additions to the continental crust volume. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A basal detachment zone recognised in the Palaeoproterozoic Peräpohja Belt, Northern Finland. • Close relationship between the Proterozoic cover and the Archaean basement. • Structural inheritance, hard- and soft-linked relationships. • Horst-graben topography of the basement attributed to rifting initiated at 2.45 Ga. • Structural control over mineralization. Abstract Reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold-and-thrust belt structural styles and patterns of the entire Palaeoproterozoic Peräpohja Belt in northern Finland show that both the deposition of the 2.4–1.9 Ga supracrustal rocks and the structural evolution during the subsequent Svecofennian Orogeny (1.9–1.8 Ga) have been tightly controlled by (i) the palaeo-topography of the Archaean (>2.5 Ga) basement, and (ii) the reactivation of Archaean basement faults during the multiple stages of rifting and deposition of the supracrustals, as well as during subsequent basin inversion and fold-and-thrust belt development. Folds and thrusts in the supracrustal sequence are found to be largely detached from the underlying basement along the Petäjäskoski Detachment Zone (PDZ) that formed within a mechanically weak stratigraphic unit in the basal part of the Peräpohja Belt stratigraphy. The pattern of contractional structures is complex, but developed during one phase of simple, progressive south-vergent thrusting. Two styles of basement-cover linkage are recognised: The hard-linked style is best illustrated by the reverse Sihtuuna Fault currently defining the northern margin of the E-W to ENE-WSW trending basement horsts and the overlying anticlines, and juxtaposing the stratigraphically lowermost Peräpohja Belt rocks with higher-up stratigraphy, indicating the fault is inherited after a pre-existing basement structure. Based on the distribution and thickness variation of the Palaeoproterozoic supracrustal units, the faults bounding the basement horst were active as extensional faults during deposition. The soft-linked style relationship is evident as abrupt terminations of thrust-related elongate domes and localisation of the compressional overprint into the NW-SE trending central graben. Understanding the basement architecture and basement-cover linkages is highly significant because the majority of the known mineralizations of the belt are located along or close to the basement faults. [ABSTRACT FROM AUTHOR]

Abstract Crust in the central Midcontinent of the USA, part of North America's intracratonic platform, consists of Precambrian crystalline basement overlain by a cover of Phanerozoic sedimentary strata. Analysis of EarthScope long-period magnetotelluric (MT) data provides an opportunity to characterize lithosphere-scale structures of this region. Inversion of these data reveals a distinct belt of high electrical conductivity (≤30 Ωm) traversing Missouri. This belt strikes northwest-southeast, dips steeply to the northeast, and extends at least from the mid-crust into the lithospheric mantle. Significantly, this "Missouri high-conductivity belt" (MHCB) spatially coincides with the Missouri Gravity Low (a distinctive feature on Bouguer anomaly maps), lies adjacent to the southwestern edge of Precambrian bedrock exposure in the Ozark Plateau, trends parallel to both magnetic-anomaly lineaments and mapped fault traces, and aligns with lateral steps (transfer or transform faults) in the Midcontinent rift and the Reelfoot rift. We suggest that the MHCB initiated as a lithosphere-scale transtensional shear zone, which linked rift segments that were active during Proterozoic extensional events. Conceivably, pull-aparts within this shear zone localized felsic igneous activity and produced depressions that filled with sediment in association with the development of the 1.5–1.3 Ga Eastern Granite-Rhyolite Province, yielding a region of less-dense crust that could account for the Missouri Gravity Low. The shear zones likely also provided pathways for fluid migration. High conductivity in the MHCB may reflect deep crustal and lithospheric mantle alteration in response to the passage of fluids during dilatational shearing. We postulate that fluid movement accompanied by shear resulted in graphite and perhaps sulfide concentration along the shear zone in the lower crust, and hydration and/or grain diminution and crystallographic preferred orientation in the lithospheric mantle. This alteration also may have weakened lithosphere along the MHCB, explaining the spatial association of the belt with Paleozoic reactivated faults and with contemporary seismicity. Highlights • Magnetotelluric imaging indicates a zone of elevated conductivity in Missouri. • The "Missouri High Conductivity Belt" (MHCB) strikes NW-SE and has a steep NE dip. • The MHCB coincides with the Missouri Gravity Low and other structures. • Interpreted as a fossilized lithosphere penetrating transtensional shear zone [ABSTRACT FROM AUTHOR]

The results of U–Pb-isotopic dating of detrital zircons from sandstones of the Proterozoic Pogor'uy Formation, occurring to the north and south of the East Angara block of Yenisei Ridge, are presented. The sandstones of the northern part of the East Angara block are dominated by Archean and Paleoproterozoic populations of detrital zircons, while the samples from the southern part of the East Angara block have been revealed up to 25% of Mesoproterozoic age grains. The detrital zircon ages allow limiting the maximum sedimentary age of the Pogor'uy Fm. and the entire interval of its formation to 1.2–0.9 Ga. The paleogeographic features of the Pogor'uy Fm. deposition in the context of the existing opinions on the location of Siberia within the Rodinia supercontinent were examined. [ABSTRACT FROM AUTHOR]

Whether the North Qinling Terrane (NQT) was accreted to the North China Craton (NCC) in the Proterozoic is still a matter of debate. We report the first detrital zircon study from the Baishugou Formation, which forms the uppermost part of the Mesoproterozoic Guandaokou Group, at the southernmost NCC margin. Detrital zircons from carbonaceous silty phyllite in the lower part of the Baishugou Formation yield U-Pb ages peaking at ca. 2500 Ma, with minor peaks at ca. 2300-2000, 1800, and 1600 Ma, and εHf(t) values ranging from −10.8 to +9.1. These zircons are considered to have been sourced from the NCC. In contrast, the middle-to-upper part of the formation contains detrital zircons which yield an age group ranging from 1800 to 1000 Ma, with peaks at 1800, 1500, 1300, and 1100 Ma; the zircons with ages of 1500-1300 Ma dominantly have εHf(t) values greater than +5 and the majority plot along the depleted mantle evolution curve. The striking difference between the U-Pb ages of the detrital zircons from the upper and lower parts of the formation suggests a shift in provenance. Magmatism at 1500-1300 Ma has not been reported from the southern margin of the NCC but has been discovered in the NQT. Hence, we deduce that the zircons from the upper part of the formation were primarily derived from the NQT, where an episode of crustal growth and magmatism is recorded between 1500 and 1100 million years. The variable sediment provenances imply that the NCC and NQT could be connected during the late Mesoproterozoic to early Neoproterozoic. The pattern of detrital zircon ages in the new sediments from the Baishugou Formation is distinct from those in the Kuanping Group and the Palaeozoic Erlangping Complex, which are at present sandwiched between the NCC and the NQT. The detrital zircons from these two groups are dominated by an age peak at ca. 1000 Ma, which is formed as the result of amalgamation of the NQT and the Rodinia Supercontinent during the Grenville orogeny. It is possible that the new sediments of the Baishugou Formation were deposited before Grenville orogeny. [ABSTRACT FROM AUTHOR]

Abstract The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Várzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D 1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D 2 and D 3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D 1 structure is an alternation of non-mylonitic S 1 and mylonitic-S 1 , developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L 1) measurements along a great circle in stereoplots is interpreted to result from shearing along S 1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma). Graphical abstract Image 1 Highlights • Studied metavolcanic rocks bearing a geochemical inprint of arc magmatism. • Three deformations phases affected the studied rocks. • Two contractional stage occurred in the Porongos Metamorphic Complex. • The studied rocks reached the metamorphic peak at amphibolite facies conditions. • Porongos and Várzea do Capivarita Complexes possibly shared a single basin. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The Shachang rapakivi granite complex was formed during 1700–1682 Ma. • Temperature and pressure decrease but fO 2 increase during crystallization. • Source of the complex is similar to lower continental crust of the NCC. Abstract Typical Proterozoic rapakivi granite of the Northern Hemisphere occur in Precambrian shield areas, and are commonly associated with anorthosites, mangerites, and charnockites, thus belonging to the AMCG (anorthosite–mangerite–charnockite–granite) suite. A similar suite is represented in the North China Craton (NCC) by the Shachang rapakivi granite and Damiao anorthosite complexes. The Shachang complex consists of amphibole–biotite rapakivi granite, porphyritic biotite granite, coarse-grained biotite granite, medium- to fine-grained biotite granite, and two-mica granite (listed from the earliest to the latest phases). New zircon U–Pb dating shows that the complex was formed over the period 1700–1682 Ma. The mafic minerals of the complex are calcic amphibole (ferroedenite–ferrohastingsite) and biotite (annite–siderophyllite), and the compositions of the amphibole suggest that the matrix of the amphibole–biotite rapakivi granite crystallized at a pressure of ∼4 kbar. The temperatures of crystallization, obtained from the compositions of amphibole, biotite, and zircon, decreased from ∼780 °C for the amphibole–biotite rapakivi granite to ∼660 °C for the two-mica granite. Our study of the Shachang complex indicates an evolution from early potassium-rich magmas to late silicon-rich magmas. The early amphibole–biotite rapakivi granite crystallized under relatively high temperatures (760–780 °C) and low fO 2 , while the later rocks formed under lower temperatures (760–660 °C) and high fO 2. The Shachang rapakivi granite and Damiao anorthosite complexes contain different mafic minerals and show diverse trace element ratios and disparate geochemical evolutionary trends, suggesting different sources. Zircon Lu–Hf and whole-rock Sm–Nd isotopic data for the Shachang complex are relatively homogeneous, with ε Hf (t) and ε Nd (t) values varying from −9.8 to −3.3 and −6.4 to −3.4, respectively. Corresponding two-stage Hf model ages and Nd depleted mantle model ages vary from 2650 to 2850 Ma and from 2480 to 2840 Ma, respectively. These model ages are similar to the ages of Neoarchean lower crustal metamorphic rocks in the NCC (2500–2800 Ma). In addition, the compositions of the amphiboles and biotites in the granite in the Shachang complex indicate derivation from the crust. All the evidence points to the Shachang rapakivi granite complex having been formed from magmas derived from the lower crust of the NCC, rather than from the fractionation of mantle-derived mafic magmas with minor crustal contamination. This paper provides a detailed case study of the Shachang complex that will further our understanding of the sources of Proterozoic rapakivi granite in the Northern Hemisphere that belong to the AMCG suite of rocks. [ABSTRACT FROM AUTHOR]

The Alxa block is located in the southwestern margin of the North China Craton. The Paleoproterozoic tectonic evolution, crustal growth and tectonic affinity of the block remain unknown or controversial. The Longshoushan (LS) area is one of the few areas that outcrop Paleoproterozoic to crystalline basement rocks in the Alxa Block. In this study, we preset whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotope data from metagabbro, metadiorite, quartz syenite, granitic leucosome and pegmatoid leucosome in the LS area. These rocks all are enriched in LREE and LILE, and depleted in HREE and HFSE. Eight new LA-ICP-MS zircon U-Pb ages yielded three magmatic ages of 2044 Ma, 2029 Ma and 1940 Ma, and three metamorphic ages of 1891 Ma, 1848 Ma and 1812 Ma. Lu-Hf analyses reveal that the magmatic zircons and anatectic/metamorphic zircons from all the rock types are characterized by positive eHf(t) (-0.16 to 10.89) and variable eHf(t) (-11.21 to 6.24), respectively. Based on the previous studies and our new data, we conclude that the LS area experienced three magmatic events (2.5-2.45 Ga, ~2.1-2.0 Ga and ~1.95-1.91 Ga) and three regional metamorphism/anataxis events (~1.93-1.89 Ga, ~1.86-1.84 Ga and ~1.81 Ga) in Paleoproterozoic. The age-Hf isotope data establishes two main crustal growth events at ~2.9-2.5 Ga and ~2.2-2.0 Ga in the LS area. These data indicate that the LS area experienced intraplate extensional setting in the middle Paleoproterozoic, and continental subduction, collision and exhumation in the late Paleoproterozoic. Combining the geochronological framework and tectonic evolution, we suggest that the Alxa Block is part of the Khondalite Belt. [ABSTRACT FROM AUTHOR]

The paleogeographic evolution of the Lake Mead region of southern Nevada and northwest Arizona is crucial to understanding the geologic history of the U.S. Southwest, including the evolution of the Colorado Plateau and formation of the Grand Canyon. The ca. 25-17 Ma Rainbow Gardens Formation in the Lake Mead region, the informally named, roughly coeval Jean Conglomerate, and the ca. 24-19 Ma Buck and Doe Conglomerate southeast of Lake Mead hold the only stratigraphic evidence for the Cenozoic pre-extensional geology and paleogeography of this area. Building on prior work, we present new sedimentologic and stratigraphic data, including sandstone provenance and detrital zircon data, to create a more detailed paleogeographic picture of the Lake Mead, Grand Wash Trough, and Hualapai Plateau region from 25 to 18 Ma. These data confirm that sediment was sourced primarily from Paleozoic strata exposed in surrounding Sevier and Laramide uplifts and active volcanic fields to the north. In addition, a distinctive signal of coarse sediment derived from Proterozoic crystalline basement first appeared in the southwestern corner of the basin ca. 25 Ma at the beginning of Rainbow Gardens Formation deposition and then prograded north and east ca. 19 Ma across the southern half of the basin. Regional thermochronologic data suggest that Cretaceous deposits likely blanketed the Lake Mead region by the end of Sevier thrusting. Post-Laramide northward cliff retreat off the Kingman/Mogollon uplifts left a stepped erosion surface with progressively younger strata preserved northward, on which Rainbow Gardens Formation strata were deposited. Deposition of the Rainbow Gardens Formation in general and the 19 Ma progradational pulse in particular may reflect tectonic uplift events just prior to onset of rapid extension at 17 Ma, as supported by both thermochronology and sedimentary data. Data presented here negate the California and Arizona River hypotheses for an "old" Grand Canyon and also negate models wherein the Rainbow Gardens Formation was the depocenter for a 25-18 Ma Little Colorado paleoriver flowing west through East Kaibab paleocanyons. Instead, provenance and paleocurrent data suggest local to regional sources for deposition of the Rainbow Gardens Formation atop a stripped low-relief western Colorado Plateau surface and preclude any significant input from a regional throughgoing paleoriver entering the basin from the east or northeast. [ABSTRACT FROM AUTHOR]

This study aims to understand the origin of giant magnesite and talc deposits in the Liaohe Group (Liaoning Province, NE China). Magnesite stromatolites and the composition of fluid inclusions suggest that magnesite or high-Mg calcite precipitated directly from strongly restricted seawater pools with meteoric influx. A primary evaporitic origin is also indicated for parts of the investigated dolomites by comparably heavy δ 18 O values. Later, intense metasomatic activity led to the formation of a magnesite/dolomite succession with irregular contacts and a lighter isotopic signature of oxygen. A slight shift in δ 18 O to more positive values was observed for talc-hosting magnesite, which can be explained by the incorporation of isotopically light oxygen into talc. This highlights that the hydrothermal processes that led to talc formation influenced the hosting carbonates as well, which is also documented by a tendency to smaller crystal sizes, a higher whiteness and lower trace element concentrations in samples from locations nearby large talc bodies. Although δ 13 C is suggested to be less sensitive to hydrothermal activity, comparably light δ 13 C values were determined for magnesite sinters, as well as for remobilized magnesites. In general, the δ 13 C signature of the investigated magnesites is lighter than expected for Proterozoic carbonates. A single-stage generation of the giant talc deposit in the study area is suggested based on elemental and isotopic data. Later deformation led to a (iso-chemical) re-location of talc at least once. During this process, irregularly distributed, cloudy/massive talc bodies acted as weak zones and were incorporated into shear bands up to several meters in thickness, which form the actually present, structurally controlled deposit. The original ore type is preserved only in areas with minor deformation. Lamprophyre dykes prove Jurassic volcanism and are clearly younger than the main phase of talc generation. A younger (post-Jurassic?), intense tectonic event is indicated by strongly tectonized dyke material that is re-worked into strike-slip faults and shows siliceous contact zones to the surrounding magnesite. These faults occasionally cross-cut the older, deposit-forming talc shear bands, indicating that the event that led to the deformation of lamprophyre dykes was younger than the main phase of talc re-location. However, this younger event did not lead to a second phase of major talc generation or affect the quality of the initial deposit to a large extent. [ABSTRACT FROM AUTHOR]

The Tarim Block contains important information of the Precambrian crustal evolution of the Earth. In this paper, we present detailed petrographic and in situ zircon U–Pb and Hf isotopes data for the Palaeoproterozoic host rocks (gneiss and gneissic granite) of the Shuangfengling gold deposit in the Quruqtagh terrane in order to constrain their ages and magma sources and discuss their tectonic environments. Zircon laser ablation–inductively coupled plasma–mass spectrometry data indicated that the host rocks (gneiss and gneissic granite) of the Shuangfengling gold deposit were formed at 1948 ± 17 and 1944 ± 13 Ma, respectively, and all underwent a metamorphism immediately after magma emplacement. The Hf isotopic signatures, negative εHf(t) values and older Archean two‐stage Hf model ages, of the studied rocks suggested that the magmas of these rocks were derived by crustal reworking in a manner similar to the magmatism related to a collisional orogeny. This study further confirms the existence of this Palaeoproterozoic magmatic and metamorphic event in the Tarim Block. The Tarim Block was part of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

The Tarim Craton is overlain by thick Neoproterozoic sedimentary successions in rift tectonic setting. This study examines the latest outcrop, seismic, and drilling core data with the objective of investigating the regional stratigraphy to deeply recognize the evolution of rifting in the craton. Cryogenian to Lower Ediacaran successions are mainly composed of clastic rocks with thicknesses of 2000–3000 m, and the Upper Ediacaran successions are composed of carbonate rocks with thicknesses of 500–800 m. The rift basins and stratigraphic zones are divided into northern and southern parts by a central paleo-uplift. The northern rift basin extends through the northern Tarim Craton in an E–W direction with two depocenters (Aksu and Kuruktag). The southern rift basin is oriented NE–SW. There are three or four phases of tillites in the northern zone, while there are two in the southern zone. Given the north–south difference of the stratigraphic framework, the northern rift basin initiated at ca. 740 Ma and the southern rift basin initiated at ca. 780 Ma. During the Cryogenian and Ediacaran, the northern and southern rift basins were separated by the central paleo-uplift, finally connecting with each other in the early Cambrian. Tectonic deformation in the Late Ediacaran led to the formation of a parallel unconformity in the rift basins and an angular unconformity in the central paleo-uplift. The Neoproterozoic rift basins continued to affect the distribution of Lower Cambrian hydrocarbon source rocks. The north–south distribution and evolution of the rift basins in the Tarim Craton have implications for reconstructions of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

The Sahiya and Khashabi volcano-sedimentary successions are exposed near the southern tip of the Sinai Peninsula, the northernmost segment of the Arabian-Nubian Shield (ANS). These Neoproterozoic successions include a series of intermediate to acidic lavas and associated pyroclastic deposits. Field observations and geochemical data reveal two distinct eruptive phases. The lavas representing each phase are intercalated with volcaniclastic greywackes and siltstones. The first eruptive phase, well exposed at Wadi Sahiya, includes basaltic andesite, andesite and dacite with minor rhyolite. The rocks of this sequence are at most weakly deformed and slightly metamorphosed. The second eruptive phase, well exposed at Wadi Khashabi, includes only undeformed and unmetamorphosed dacite and rhyolite. The two volcano-sedimentary successions were separated and dismembered during intrusion of post-collisional calc-alkaline and alkaline granites. Geochemical compositions of the Sahiya and Khashabi volcanic rocks confirm the field data indicating discrete phases of magmatism, however all the compositions observed might plausibly be derived from a common source and be related to one another dominantly through fractional crystallization. The low and variable Mg# values (55–33) measured in the basaltic andesites and andesites preclude their equilibration with a mantle source. Rather, even the most primitive observed lavas are already the products of significant fractional crystallization, dominated by removal of amphibole and plagioclase. Continued fractionation eventually produced dacite and rhyolite marked by significant depletion in Y and HREE. The gradual appearance of negative Nb-Ta anomalies with increasing SiO 2 through both suites suggests at least some component of progressive crustal contamination. The medium- to high-K calc-alkaline character of the Sahiya and Khashabi volcanics could be explained either by their formation at an active continental margin or by a two-stage model that appeals to re-melting of arc material in a post-collisional setting. The Wadi Sahiya basaltic andesite and andesite samples exhibit the defining chemical characteristics of adakites: high Sr (>700 ppm), low Y (<16 ppm), high Sr/Y (>20) and low Yb (<1.8 ppm). Although this signature can be associated with slab melting, here we show that it reflects partial melting of lithospheric mantle beneath thickened continental arc crust. The early eruptive phase, exposed at Sahiya, was erupted on an active continental margin, whereas the later Khashabi succession marks the transition to a post-collisional stage. [ABSTRACT FROM AUTHOR]

Cassiterite (SnO 2 ), a main ore mineral in tin deposits, is suitable for U–Pb isotopic dating because of its relatively high U/Pb ratios and typically low common Pb. We report a LA-ICPMS analytical procedure for U–Pb dating of this mineral with no need for an independently dated matrix-matched cassiterite standard. LA-ICPMS U-Th-Pb data were acquired while using NIST 612 glass as a primary non-matrix-matched standard. Raw data are reduced using a combination of Iolite™ and other off-line data reduction methods. Cassiterite is extremely difficult to digest, so traditional approaches in LA-ICPMS U-Pb geochronology that utilize well-characterized matrix-matched reference materials (e.g., age values determined by ID-TIMS) cannot be easily implemented. We propose a new approach for in situ LA-ICPMS dating of cassiterite, which benefits from the unique chemistry of cassiterite with extremely low Th concentrations (Th/U ratio of 10 −4 or lower) in some cassiterite samples. Accordingly, it is assumed that 208 Pb measured in cassiterite is mostly of non-radiogenic origin—it was initially incorporated in cassiterite during mineral formation, and can be used as a proxy for common Pb. Using 208 Pb as a common Pb proxy instead of 204 Pb is preferred as 204 Pb is much less abundant and is also compromised by 204 Hg interference during the LA-ICPMS analyses. Our procedure relies on 208 Pb/ 206 Pb vs 207 Pb/ 206 Pb (Pb-Pb) and Tera-Wasserburg 207 Pb/ 206 Pb vs 238 U/ 206 Pb (U-Pb) isochron dates that are calculated for a ~1.54 Ga low-Th cassiterite reference material with varying amounts of common Pb that we assume remained a closed U-Pb system. The difference between the NIST 612 glass normalized biased U-Pb date and the Pb-Pb age of the reference material is used to calculate a correction factor (F) for instrumental U-Pb fractionation. The correction factor (F) is then applied to measured U/Pb ratios and Tera-Wasserburg isochron dates are obtained for the unknown cassiterite analyzed in the same analytical session. This allows for U-Pb dating of cassiterite of any age with no need for an independently dated matrix-matched reference material, nor assumptions about the isotopic composition of common Pb. Results for cassiterite from tin deposits in Bolivia, Brazil, China, Russia, Saudi Arabia, South Africa, Spain, and the United Kingdom, with ages ranging from ~20 Ma to ~2060 Ma, demonstrate the applicability of this approach across a broad range of geologic time. These ages are in good agreement with published geochronology of the host rocks associated with the tin deposits and with previously published U-Pb ages of some cassiterites from the same deposits. Thus, our in situ LA-ICPMS methodology verifies the use of cassiterite as a reliable U-Pb mineral-geochronometer with the advantages of fast and relatively low cost in situ analyses with moderate spatial resolution. [ABSTRACT FROM AUTHOR]

Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U-Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca-Mojave province to the west, characterized by the so-called Bámori Complex, have U-Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved εHf values, yielding TDM ages of ca. 2.1-2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile εHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile εHf values and a TDM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile εHf values ranging from −1 to +8 and TDM dates of ca. 1.8-1.9 Ga and 1.6-1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave-Sonora megashear. [ABSTRACT FROM AUTHOR]

Reflectance spectroscopy is a rapid, non-destructive technique capable of characterizing mineral and organic components within geologic materials at spatial scales that range from μm to km. The degree to which reflectance spectra can be used to provide quantitative information about organic compounds remains poorly understood, particularly for rocks with low organic content that are common in the Earth's ancient rock record and that may be present on other planetary bodies, such as Mars. In the present study, reflectance spectra (0.35–25 μm) were acquired for a suite of Proterozoic shales and the kerogen was isolated to assess how spectral properties of aliphatic and aromatic C-H absorption bands can be used to predict organic matter abundance (total organic content, TOC, and H/C ratio). A number of spectral parameters were evaluated for organic absorption bands observed in the 3–4 μm wavelength region for comparison with independently measured TOC and H/C values. Ratios of the strengths of aliphatic to aromatic absorption bands were directly correlated to H/C values, but the reflectance spectra for pure kerogens with H/C < 0.2 lacked clear evidence for C-H absorption bands in this spectral region. Organic absorption bands are routinely observed for bulk rock powders with <1 wt.% TOC, but the detection limits of reflectance spectra for TOC may be >1 wt.% or as high as 10 wt.%. Organic detection limits for reflectance spectra are, thus, controlled by both TOC and H/C values, but these parameters can be predicted for clay-rich, kerogen-dominated samples for a range of values that are relevant to drill cores, outcrops, meteorites, and planetary surfaces. [ABSTRACT FROM AUTHOR]

Granites derived from partial melting of sedimentary rocks are generally characterized by high δ 18 O values and abundant relict zircons. Such relict zircons are valuable in tracing the source rocks of granites and the history of crustal anatexis. Here we report in-situ U-Pb ages, O isotopes and trace elements in zircons from Triassic granites in the Zhuguangshan and Jiuzhou regions, which are located in the Nanling Range and the Darongshan area, respectively, in South China. Zircon U-Pb dating yields magma crystallization ages of 236 ± 2 Ma for the Zhuguangshan granites and 246 ± 2 Ma to 252 ± 3 Ma for the Jiuzhou granites. The Triassic syn -magmatic zircons are characterized by high δ 18 O values of 10.1–11.9‰ in Zhuguangshan and 8.5–13.5‰ in Jiuzhou. The relict zircons show a wide range of U-Pb ages from 315 to 2185 Ma in Zhuguangshan and from 304 to 3121 Ma in Jiuzhou. Nevertheless, a dominant age peak of 700–1000 Ma is prominent in both occurrences, demonstrating that their source rocks were dominated by detrital sediments weathered from Neoproterozoic magmatic rocks. Taking previous results for regional granites together, Neoproterozoic relict zircons show δ 18 O values in a small range from 5 to 8‰ for the Nanling granites but a large range from 5 to 11‰ for the Darongshan granites. In addition, relict zircons of Paleozoic U-Pb age occur in the two granitic plutons. They exhibit consistently high δ 18 O values similar to the Triassic syn -magmatic zircons in the host granites. These Paleozoic relict zircons are interpreted as the peritectic product during transient melting of the metasedimentary rocks in response to the intracontinental orogenesis in South China. Therefore, the relict zircons of Neoproterozoic age are directly inherited from the source rocks of S-type granites, and those of Paleozoic age record the transient melting of metasedimentary rocks before intensive melting for granitic magmatism in the Triassic. [ABSTRACT FROM AUTHOR]

The Blackbush uranium deposit (JORC Inferred Resource: 12,580 tonnes U), located on the north-eastern Eyre Peninsula, is currently the only sediment-hosted U deposit investigated in detail in the Gawler Craton. Uranium is hosted within Eocene sandstone of the Kanaka Beds, overlying Mesoproterozoic granites of the Samphire pluton, affiliated with the Hiltaba Intrusive Suite (~ 1.6 Ga). These are considered the most probable source rocks for uranium mineralisation. By constraining the petrography and mineralogy of the granites, insights into the post-emplacement evolution can be gained, which may provide an exploration indicator for other sediment-hosted uranium systems. Three geochemically distinct granite types were identified in the Samphire Pluton and correspond to domains interpreted from geophysical data. All granites show complex alteration overprints and textures with increasing intensity closer to the deposit, as well as crosscutting veining. Alkali feldspar has been replaced by porous K-feldspar and albite, and plagioclase is overprinted by an assemblage of porous albite + sericite ± calc-silicates (prehnite, pumpellyite and epidote). This style of feldspar alteration is regionally widespread and known from Hiltaba-aged granites associated with iron-oxide copper‑gold mineralisation at Olympic Dam and in the Moonta-Wallaroo region. In two granite types biotite is replaced by calcic garnet. Calc-silicates are indicative of Ca-metasomatism, sourced from the anorthite component of altered plagioclase. Minor clay alteration of feldspars is present in all samples. Mineral assemblages in veins include quartz + hematite, hematite + coffinite, fluorite + quartz, and clay minerals. Minor chlorite and sericite are found in all vein types. All granite types are anomalously rich in U (concentrations between 10 and 81 ppm). Highly variable Th/U ratios, as well as hydrothermal U minerals (mostly coffinite) in granites and veins, are clear evidence for U mobility. Uranium may have been preconcentrated in veins in the upper parts of the pluton, and was subsequently leached after deposition of the sediment. [ABSTRACT FROM AUTHOR]

In this paper, U-Pb zircon geochronology, Lu-Hf and Sm-Nd isotope systematics, geochemistry and geothermobarometry of metaigneous basement rocks exposed in the southeastern Chiapas Massif Complex are presented. Geologic mapping of the newly defined “El Triunfo Complex” located at the southeastern edge of the Chiapas Massif reveals (1) partial melting of a metamorphic basement mainly constituted by mafic metaigneous rocks (Candelaria unit), (2) an Ediacaran metasedimentary sequence (Jocote unit), and (3) occurrence of massif-type anorthosite. All these units are intruded by undeformed Ordovician plutonic rocks of the Motozintla suite. Pressure and temperature estimates using Ca-amphiboles, plagioclase and phengite revealed prograde metamorphism that reached peak conditions at ~ 650 °C and ~ 6 kbar, sufficient for partial melting under water saturated conditions. Relict rutile in titanite and clinopyroxene in amphibolite further indicate a previous metamorphic event at higher P-T conditions. U-Pb zircon ages from felsic orthogneiss boudins hosted in deformed amphibolite and migmatite yield crystallization ages of ~ 1.0 Ga, indicating that dry granitic protoliths represent remnants of Rodinia-type basement. Additionally, a mid-Tonian (~ 920 Ma) metamorphic overprint is suggested by recrystallized zircon from a banded gneiss. Zircon from folded amphibolite samples yield mainly Ordovician ages ranging from ~ 457 to ~ 444 Ma that are indistinguishable from the age of the undeformed Motozintla plutonic suite. Similar ages between igneous- and metamorphic- zircon suggest a coeval formation during a high-grade metamorphic event, in which textural discrepancies are explained in terms of differing zircon formation mechanisms such as sub-solidus recrystallization and precipitation from anatectic melts. In addition, some amphibolite samples contain inherited zircon yielding Stenian-Tonian ages around 1.0 Ga. Lu-Hf and Sm-Nd isotopes and geochemical data indicate that the protoliths of the amphibolite have E-MORB characteristics and were derived from a depleted mantle source younger than the Rodinia-type basement. Inasmuch as similar amphibolites also occur in the Ediacaran metasedimentary rocks as dykes or lenses, Late Neoproterozoic magmatism in a rift setting is suggested. Hence, the geologic record of the El Triunfo Complex includes evidences for Rodinia assemblage, Tonian circum-Rodinia subduction, and breakup during the Late Neoproterozoic. Metamorphism, and partial melting are interpreted in terms of a convergent margin setting during the Ordovician. The results place the southern Chiapas Massif along with Oaxaquia and similar Northern Andes terranes on the NW margin of Gondwana interpreted as the extension of the Famatinian orogen that evolved during the closure of the Iapetus Ocean. [ABSTRACT FROM AUTHOR]

Little Salt Spring (Sarasota County, FL, USA) is a sinkhole with groundwater vents at ~77 m depth. The entire water column experiences sulfidic (~50 μM) conditions seasonally, resulting in a system poised between oxic and sulfidic conditions. Red pinnacle mats occupy the sediment-water interface in the sunlit upper basin of the sinkhole, and yielded 16S rRNA gene clones affiliated with Cyanobacteria, Chlorobi, and sulfate-reducing clades of Deltaproteobacteria. Nine bacteriochlorophyll e homologues and isorenieratene indicate contributions from Chlorobi, and abundant chlorophyll a and pheophytin a are consistent with the presence of Cyanobacteria. The red pinnacle mat contains hopanoids, including 2-methyl structures that have been interpreted as biomarkers for Cyanobacteria. A single sequence of hpnP, the gene required for methylation of hopanoids at the C-2 position, was recovered in both DNA and cDNA libraries from the red pinnacle mat. The hpnP sequence was most closely related to cyanobacterial hpnP sequences, implying that Cyanobacteria are a source of 2-methyl hopanoids present in the mat. The mats are capable of light-dependent primary productivity as evidenced by 13C-bicarbonate photoassimilation. We also observed 13C-bicarbonate photoassimilation in the presence of DCMU, an inhibitor of electron transfer to Photosystem II. Our results indicate that the mats carry out light-driven primary production in the absence of oxygen production-a mechanism that may have delayed the oxygenation of the Earth's oceans and atmosphere during the Proterozoic Eon. Furthermore, our observations of the production of 2-methyl hopanoids by Cyanobacteria under conditions of low oxygen and low light are consistent with the recovery of these structures from ancient black shales as well as their paucity in modern marine environments. [ABSTRACT FROM AUTHOR]

The Snowbird Tectonic Zone (STZ), the ca. 2800 km-long boundary between Rae and Hearne domains of the western Canadian Shield, remains of uncertain tectonic significance, but is currently interpreted as a ca. 1.9 Ga suture. The eastern Rae domain is well studied and underlain by a >20,000 km 2 region of high pressure granulites, containing ca. 2.60 Ga arc rocks, and granulite to amphibolite-grade structures, which formed at ca 2.57 Ga (D 1 /M 1 ) and 1.90 Ga (D 2 /M 2 ). The 1.85 Ga Legs Lake shear zone juxtaposed the high-P granulites of the eastern Rae domain structurally above lower amphibolite- to greenschist-grade supracrustal gneisses of the Hearne domain. Zircon U-Pb and Hf isotopic analyses were acquired from igneous and metasedimentary rocks across the STZ and Rae-Hearne boundary to assess the validity of the suture model, with specific focus given to the >5 km wide Legs Lake shear zone. Results indicate that both 2.60–2.55 Ga magmatism and deformation continue unimpeded across the STZ and Rae-Hearne boundary. Furthermore, Hf isotopic data from igneous zircon from 2.60 Ga plutonic rocks across the STZ display little variation in ε Hf indicating that no major crustal-break corresponds to the Rae-Hearne boundary, or any of the shear zones that parallel the STZ on the ground. Older plutonic rocks show a greater variability in ε Hf , which may indicate a disparate origin. Grade, intensity, and timing of Paleoproterozoic features vary along the transect and display little continuity across the Rae-Hearne domain boundary. In conjunction with existing field and isotopic data, we interpret that the Rae and Hearne domains were together by 2.60 Ga, and that the STZ was host to repeated Paleoproterozoic reactivation driven by the latter stages of Taltson-Thelon orogenesis, final indentation of the Slave craton into the Rae domain, and accretionary processes on the southern periphery of the Hearne domain from 1.93 to 1.84 Ga. Furthermore, assessing the viability of major shear zones within high grade terranes as potential sutures is complicated by the lateral and vertical heterogeneity of the crust juxtaposed by them. [ABSTRACT FROM AUTHOR]

A regional tectonic model is presented for the formation and evolution of Paleoproterozoic basins over the northern Yilgarn Craton, integrating new and published regional geological, geochronological, geochemical and geophysical data. The basins formed as a response to both extensional and compressional processes in the early Paleoproterozoic along the craton margin. Early rifting and basin formation coincided with the Ophthalmia Orogeny, the result of the convergence and accretion of the Archean to Proterozoic Glenburgh Terrane with the Pilbara Craton, and led to the formation of the Yerrida Basin at c. 2180 Ma as a single sub-basin containing the Windplain Group. This led to the eventual development of the Bryah and Mooloogool Sub-basins of the Yerrida Basin at c. 2030 Ma, and voluminous extrusion and intrusion of mafic rocks. The depth and nature of the Bryah Sub-basin suggests formation in response to rifting, with its orientation corresponding to the rift axis. Continued rifting along the northern Yilgarn margin resulted in subduction of the Yilgarn Craton beneath the composite Glenburgh–Pilbara craton. Eventual collision was marked by cessation of volcanism and rift-sediment deposition in the Bryah and Mooloogool Sub-basins, the onset of a pro-foreland basin (Padbury Basin) in the west, and approximately NE–SW rifting further east (the Earaheedy Basin), all at c. 2000 Ma. Banded iron-formation and granular iron-formation (Robinson Range and Frere Formations) was deposited much later (c. 1890 Ma) in a large basin that deepened from east to west, spanning the Earaheedy and Yerrida Basins. The deepest parts of this basin coincide with the Bryah Sub-basin, which was the most rifted portion of the Yilgarn Craton in this region. Deposition in all basins probably ceased with onset of the Capricorn Orogeny at c. 1820 Ma. [ABSTRACT FROM AUTHOR]

The present study, based on combined geophysical, geochronological and isotopic data identifies a cratonic block which we call the Western Chihuahua-Mesa Central megablock (WCMB) in northern Mexico. New Bouguer and isostatic gravity anomaly maps revealed a conspicuously distinct, spatially-extensive gravity minima over a region extending from north western Chihuahua to as far south as the Mesa Central (MC). This region can be differentiated from the surrounding mobile belts with significantly higher gravity anomaly values. Gravity modeling combined with sparse seismically determined crustal thickness estimates indicate that the proposed WCMB region has crustal thicknesses ranging from 38 to 42 km. The regions surrounding WCMB are the spatially constrained mobile belts that are characterized by basinal and trough-like structures and are associated with A-type granites and bimodal volcanism. Spatial distribution of enriched initial Sr isotopic data (>0.706) confirms the sialic character of the crust associated with the gravity minimum at least in western Chihuahua. The mobile belts surrounding the WCMB contain geologic records of the events since 1.8–0.9 Ga. These ages from within and just adjacent to the block correlate well with the Rodinia Neoproterozoic supercontinent that was assembled 1.3–0.9 Ga. We propose that the northwest-southeast directed WCMB is a cratonic region underlain by Proterozoic crust. The WCMB is most likely separated from the deformed and active southwestern part of the North American Proterozoic craton by repeated thermotectonic activities within the surrounding mobile belts. These events have helped to dislocate and transport the block to its present location through processes of repeated compression, extension, and transform faulting. [ABSTRACT FROM AUTHOR]

Magmatic “flare-ups” are common in continental arcs. The best-studied examples of such flare-ups are from Cretaceous and younger continental arcs, but a more ancient example is preserved in Late Ediacaran–Cambrian or Cadomian arcs that formed along the northern margin of Gondwana. In this paper, we report new trace-element, isotopic and geochronological data on ∼550 Ma magmatic rocks from the Taknar complex, NE Iran, and use this information to better understand episodes of flare-up, crustal thickening and magmatic periodicity in the Cadomian arcs of Iran and Anatolia. Igneous rocks in the Taknar complex include gabbros, diorites, and granitoids, which grade upward into a sequence of metamorphosed volcano-sedimentary rocks with interlayered rhyolites. Granodioritic dikes crosscut the Taknar gabbros and diorites. Gabbros are the oldest units and have zircon U–Pb ages of ca 556 Ma. Granites are younger and have U–Pb zircon ages of ca 552–547 Ma. Rhyolites are coeval with the granites, with U–Pb zircon ages of ∼551 Ma. Granodioritic dikes show two U–Pb zircon ages; ca 531 and 548 Ma. Geochemically, the Taknar igneous rocks have calc-alkaline signatures typical of continental arcs. Whole-rock Nd and zircon O–Hf isotopic data show that from Taknar igneous rocks were generated via mixing of juvenile magmas with older continental crust components at an active continental margin. Compiled geochronological and geochemical data from Iran and Anatolia allow identification of a Cadomian flare-up along northern Gondwana. The compiled U–Pb results from both magmatic and detrital zircons indicate the flare-up started ∼572 Ma and ended ∼528 Ma. The Cadomian flare-up was linked to strong crustal extension above a S-dipping subduction zone beneath northern Gondwana. The Iran–Anatolian Cadomian arc represents a site of crustal differentiation and stratification and involved older (Archean?) continental lower–middle crust, which has yet to be identified in situ , to form the continental nuclei of Anatolia and Iran. The Cadomian crust of Anatolia and Iran formed a single block “Cimmeria” that rifted away from northern Gondwana and was accreted to southern Eurasia in late Paleozoic time. [ABSTRACT FROM AUTHOR]

The ore genesis of the Paleoproterozoic iron oxide apatite deposits in the vicinity of Kiruna in northern Sweden is poorly understood, despite a century-long mining history and 2500 Mt of iron ore with grades of 30 to 70 wt% Fe produced in the region to date. Zircon grains from the ore, recently dated at ca. 1874 Ma, show very different appearances compared to zircon from surrounding host rocks (ca. 1880 Ma) and related intrusions (ca. 1880 and ca. 1874 Ma), particularly an inclusion-rich rim. In contrast, zircon from the host rocks, and a proximal granite intrusion, exhibit typical igneous growth zoning. Electron microprobe results show near stoichiometric composition for Zr, Si, and Hf in the host rock zircon grains. The ore zircon crystals have low analytical totals with significant concentrations of Ca, Fe, Y, and P and infrared spectroscopy showed several weight percent of water. These ore zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment. Transmission electron microscopy (TEM) shows that the LREE are not due to micro- or nano-inclusions in the zircon, but are likely hosted as LREE oxides in amorphous regions of the grains. Based on these characteristics, the rims on ore zircon grains are interpreted to be of hydrothermal origin. Uranium-Pb in monazite from the ore, measured by SIMS, suggests a secondary event influencing the area at ca. 1624 Ma, a period of known geologic activity in Fennoscandia. Electron microprobe X-ray mapping of these monazite grains shows no zoning and relatively low U and Th concentrations. Stark contrasts are visible between the ore (depleted mantle influence) and host rocks (crustal influence) in the whole rock Lu-Hf and Sm-Nd data. The depleted mantle signature of the ore could be related to the Kiruna greenstone group as a potential source region for the iron. The Sm-Nd isotopic composition of monazite from the ore shows a crustal influence, and indicates that the younger event has not disturbed the whole rock Sm-Nd signature of the ore. The hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation, with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source. [ABSTRACT FROM AUTHOR]

Late Vendian (540-550 Ma) U-Pb age was established for zircon from postcollisional granites of the Osinovsky Massif located among island-arc complexes of the Isakovka terrane in the northwestern Sayan-Yenisei accretionary belt. The granites were formed 150 Ma after the formation of the host island-arc complexes and 50-60 Ma after the beginning of their accretion to the Siberian Craton. These events mark the final stage of the Neoproterozoic history of the Yenisei Ridge related to the end of accretion of oceanic fragments and the beginning of the Caledonian Orogeny. The granites are subalkaline leucoractic Na-K rocks enriched in Rb, U, and Th. The petrogeochemical and Sm-Nd isotope data (T(DM)-2st = 1490-1650 Ma and ε(T) from-2.5 to-4.4) indicate that their source was highly differentiated continental crust of the SW margin of the Siberian Craton. Therefore, the host Late Riphean island-arc complexes were thrust over the craton margin for distance significantly exceeding the size of the Osinovsky Massif. [ABSTRACT FROM AUTHOR]

The study area situated east of the intracratonic Proterozoic Cuddapah basin, that covers Nellore Schist belt, Ongole domain of the Eastern Ghats Belt and adjacent East Coast Terrain, forms a geodynamically important segment of the Dharwar craton. This region came into focus due to proposition that during Paleoproterozoic to Mesoproterozoic era, it underwent persistent oceanic subduction, continent-continent collision and multi-stage accretional growth. In view of this, we have reprocessed in detail the deep seismic sounding data over this region and integrated the obtained results with other geophysical studies to decipher the deep subsurface crustal structure underneath. The derived shallow seismic section across this region showed the presence of a thin layer (~250 m) of Gondwana sediments with a distinct velocity of 4.20 km/s and a lateral dimension of about 40 km around Vinjamuru region of the Nellore Schist Belt. Another important finding has been the depiction of 5.3 and 5.5 km/s velocity sediments (comparable to upper and lower Cuddapah sediments) between Nellore Schist Belt and the east coast, indicating the possible presence of an additional Proterozoic sedimentary terrain (named here as East Coast Sedimentary Basin), overlying the concealed Archean crystalline basement. The presence of this sedimentary terrain, which is separated from the Cuddapah basin by an exhumed horst-like feature, situated below Nellore Schist Belt, would indicate Proterozoic sedimentation in a much larger area during Columbia and Rodinia supercontinental assembly periods than hitherto known. Importantly, no seismic trace of the Eastern Ghats Belt has been found between Nellore Schist Belt and the east coast. It appears that during the Gondwana period, entire eastern segment of the east Dharwar craton has been persistently rifting, uplifting and eroding together with other Gondwana grabens in India, thereby leaving behind only a thin veneer of Gondwana sediments at many places. In view of the presence of number of Gondwana occurrences all along the east coast, it is felt that the rifting during the Gondwana period may have been a prelude to India-Antarctica breakup, with final separation between the two taking place much later during early Cretaceous. [ABSTRACT FROM AUTHOR]

Great Sand Dunes National Park and Preserve (GRSA, Colorado) lies along the eastern margin of the San Luis Basin and the tectonically active Sangre de Cristo fault system that are part of the northern Rio Grande rift. GRSA lies within a prominent embayment in the range front where two separate sections of the Sangre de Cristo fault system intersect. Fault scarps are observed along both intersecting fault zones within older basin-fill alluvium, but have been obscured by the actively migrating dunefield. The dune sand is also strongly magnetic, locally limiting the usefulness of aeromagnetic methods for mapping concealed structure. This study uses airborne geophysical methods, primarily airborne gravity gradient data, along with constraints from geologic mapping and limited subsurface data and groundwater modeling, to interpret the subsurface basin geometry and range-front structure of the embayment. Using forward modeling of the gravity gradient data and locations of faults inferred from gravity gradient and aeromagnetic lineaments, several previously unrecognized tectonic elements are interpreted adjacent to the range front. Some of the largest rift-related fault offsets are demonstrated to be basinward of the normal fault zones mapped at the surface along the range front of the Sangre de Cristo Mountains, along faults concealed under the dunefield and subparallel to the two fault sections. A fault-bounded structural bench, likely composed of Proterozoic rocks, underlies most of the high dunefield at depths of 500 m to 1 km. The bench is truncated on its southwest margin by a northwest-trending, southwest-dipping normal fault. A northeast-trending, northwest-dipping normal fault with ~600 m of estimated relief lies under the southern margin of the dunefield and bounds a structurally higher bench of Proterozoic rocks concealed at <400 m depth near the range front. The northwest- and northeast-trending geophysical lineaments generally correspond well with the trends of faults mapped at the surface, and with both pre- and syn-rift structures in the Sangre de Cristo Mountains. Aeromagnetic anomalies are explained by variations in the magnetization of pre-rift rocks, and the strongly magnetic dune sand. [ABSTRACT FROM AUTHOR]

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle–crustal source and magmatic evolution. We analyzed Lu–Hf isotopic compositions of zircon from the Sancheong–Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U–Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207 Pb/ 206 Pb ages of 1870 ± 2 Ma and 1861 ± 6 Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207 Pb/ 206 Pb zircon ages of 1861 ± 6 Ma, 1872 ± 6 Ma, and 1873 ± 4 Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862 ± 2 Ma), are indicative of episodic AMCG magmatism over an ∼10 Ma interval. Initial ε Hf (t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to −6.1 and −0.3 to −5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr–Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87–1.86 Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent. [ABSTRACT FROM AUTHOR]

The Meghalaya Plateau and the Mikir Hills constitute a northeastern extension of the Precambrian Indian Shield. They are dominantly composed of Proterozoic basement granite gneisses, granites, migmatites, granulites, the Shillong Group metasedimentary cover sequence, and Mesozoic-Tertiary igneous and sedimentary rocks. Medium to coarse grained, equigranular to porphyritic Cambrian granite plutons intrude the basement granite gneisses and the Shillong Group. U-Pb SHRIMP zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism, supercontinent cycles, and crustal growth of the Meghalaya Plateau and Mikir Hills. Zircons from the Rongjeng granite gneiss record the oldest magmatism at 1778 ± 37 Ma. An inherited zircon core has an age of 2566.4 ± 26.9 Ma, indicating the presence of recycled Neoarchaean crust in the basement granite gneisses. Zircons from the Sonsak granite have two ages: 523.4 ± 7.9 Ma and 1620.8 ± 9.2 Ma, which indicate partial assimilation of an older granite gneiss by a younger granite melt. Zircons from the Longavalli granite gneiss of the Mikir Hills has a crystallization age of 1430.4 ± 9.6 Ma and a metamorphic age of 514 ± 18.6 Ma. An inherited core of a zircon from Longavalli granite gneiss has an age of 1617.1 ± 14.5 Ma. Zircons from younger granite plutons have Cambrian mean ages of 528.7 ± 5.5 Ma (Kaziranga), 516 ± 9.0 Ma (South Khasi), 512.5 ± 8.7 Ma (Kyrdem), and 506.7 ± 7.1 Ma and 535 ± 11 Ma (Nongpoh). These plutons are products of the global Pan-African tectonothermal event, and their formation markedly coincides with the later stages of East Gondwana assembly (570–500 Ma, Kuunga orogen). The older inherited zircon cores (2566.4 ± 26.9 Ma, 1758.1 ± 54.3 Ma, 1617.1 ± 14 Ma) imply a significant role for recycled ancient crust in the generation of Cambrian granites. Thus the Meghalaya Plateau and Mikir Hills experienced major felsic magmatic episodes at ~ 1800 Ma, ~ 1600 Ma, ~ 1400 Ma, and ~ 500 Ma with recycling of Neoarchaean crust, and later contributions from Paleo-Mesoproterozoic granite gneiss sources. A 258 ± 20 Ma lower intercept age of the Rongjeng granite gneiss perhaps indicates a Permo-Triassic thermal imprint on the Meghalaya Plateau. The granite gneisses and granites have peraluminous to metaluminous compositions, and syn-orogenic to post-collisional affinities. We conclude that the orogenic history of the Meghalaya Plateau and the Mikir Hills records crustal growth of the Columbia and Gondwana supercontinents as noted in other Pan-African-Indian-Prydz-Brasiliano orogens. [ABSTRACT FROM AUTHOR]

First systematic data on the variations of ε(T) in the Neoproterozoic sedimentary sequence of the Baikal-Patom fold belt (Northern Transbaikalia) are reported. The range of obtained εNd(T) is-19.4...-2.0. It was established that the rocks of the Ballaganach and most part of the Dal'nyaya Taiga groups are characterized by ε(T) from-19.4 to-16.3. Upsection, beginning from the Khomolkha Formation, the rocks show a sharp change in the initial Nd isotope composition (ε(T)-8.3...-2.0). The results of Sm-Nd study of metasedimentary rocks of the BPB, as well as the comparison of their Sm-Nd characteristics with those of the inferred source areas suggest that the input of terrigenous material at the early stage of sedimentation in the Baikal-Patom belt was mainly related to the destruction of the Early Proterozoic crust of the Siberian craton. Owing to a change of sedimentation setting from passive continental margin to the 'foreland' basin at the late stage, the terrigenous material of the Siberian craton was diluted by clastics from juvenile Neoproterozoic crust of the Baikal-Muya belt. [ABSTRACT FROM AUTHOR]

Geochemical Sm-Nd isotope and U-Th-Pb geochronological (LA-ICP-MS) studies have demonstrated that the Zeya Group of the Stanovoy Complex of the Dzhugdzhur-Stanovoy Superterrane (Central Asian fold belt) is not Paleoarchean, as was previously thought, but Paleoproterozoic in age. [ABSTRACT FROM AUTHOR]

Uranium mineralization in Bichun area, Jaipur district, Rajasthan, India is hosted by albitites within the Banded Gneissic Complex (BGC). Detailed mineralogical and EPMA studies reveal the presence of davidite along with brannerite and uraninite. The U--Pb concordia upper intercept age of 933 ± 13 Ma and Pb--Pb isochron age of 930 ± 4 Ma, on pure davidite fractions indicate the timing of uranium mineralizing event to be ca. 930 Ma. The timing of uranium mineralization can be correlated with the Grenvillian orogeny (ca. 1000 Ma). The Sm--Nd model age (TDM) of daviditeis varies from 1851 to 2200 Ma with εNdi(930 Ma) ranging from -10.7 to -15.5 which shows that the Palaeoproterozoic rocks with crustal component (either within BGC or basement granite) are the source for uranium. [ABSTRACT FROM AUTHOR]

This contribution discusses the development of the Palaeoproterozoic Buganda-Toro belt in the Rwenzori Mountains and its influence on the western part of the East African Rift System in Uganda. The Buganda-Toro belt is composed of several thick-skinned nappes consisting of Archaean Gneisses and Palaeoproterozoic cover units that are thrusted northwards. The high Rwenzori Mountains are located in the frontal unit of this belt with retrograde greenschist facies gneisses towards the north, which are unconformably overlain by metasediments and amphibolites. Towards the south, the metasediments are overthrust by the next migmatitic gneiss unit that belongs to a crustal-scale nappe. The southwards dipping metasedimentary and volcanic sequence in the high Rwenzori Mountains shows an inverse metamorphic grade with greenschist facies conditions in the north and amphibolite facies conditions in the south. Early D1 deformation structures are overgrown by cordierite, which in turn grows into D2 deformation, representing the major northwards directed thrusting event. We argue that the inverse metamorphic gradient develops because higher grade rocks are exhumed in the footwall of a crustal-scale nappe, whereas the exhumation decreases towards the north away from the nappe leading to a decrease in metamorphic grade. The D2 deformation event is followed by a D3 E-W compression, a D4 with the development of steep shear zones with a NNE-SSW and SSE-NNW trend including the large Nyamwamba shear followed by a local D5 retrograde event and D6 brittle reverse faulting. The Palaeoproterozoic Buganda-Toro belt is relatively stiff and crosses the NNE-SSW running rift system exactly at the node where the highest peaks of the Rwenzori Mountains are situated and where the Lake George rift terminates towards the north. Orientation of brittle and ductile fabrics show some similarities indicating that the cross-cutting Buganda-Toro belt influenced rift propagation and brittle fault development within the Rwenzori Mountains and that this stiff belt may form part of the reason why the Rwenzori Mountains are relatively high within the rift. [ABSTRACT FROM AUTHOR]

Basement complex of the Aravalli craton (NW India) known as the Banded Gneissic Complex (BGC) is classified into two domains viz. Archean BGC-I and Proterozoic BGC-II. We present first comprehensive geochemical study of the Archean metasedimentary rocks occurring within the BGC-I. These rocks occur associated with intrusive amphibolites in a linear belt within the basement gneisses. The association is only concentrated on the western margin of the BGC-I. The samples are highly mature (MS m ) to very immature (MS i ), along with highly variable geochemistry. Their major (SiO 2 /Al 2 O 3 , Na 2 O/K 2 O and Al 2 O 3 /TiO 2 ) and trace (Th/Sc, Cr/Th, Th/Co, La/Sc, Zr/Sc) element ratios, and rare earth element (REE) patterns are consistent with derivation of detritus from the basement gneisses and its mafic enclaves, with major contribution from the former. Variable mixing between the two end members and closed system recycling (cannibalism) resulted in the compositional heterogeneity. Chemical index of alteration (CIA) of the samples indicate low to moderate weathering of the source terrain in a sub-tropical environment. In A-CN-K ternary diagram, some samples deceptively appear to have undergone post-depositional K-metasomatism. Nevertheless, their petrography and geochemistry (low K 2 O and Rb) preclude the post-depositional alteration. We propose non-preferential leaching of elements during cannibalism as the cause of the deceptive K-metasomatism as well as enigmatic low CIA values of some highly mature samples. The Archean metasedimentary rocks were deposited on stable basement gneisses, making the BGC-I a plausible participant in the Archean Ur supercontinent. [ABSTRACT FROM AUTHOR]

A well preserved and diversified microfossil assemblage is reported from the Meso–Neoproterozoic Mbuji-Mayi Supergroup in the Kasai oriental Province, central part of Democratic Republic of Congo. A total of 49 taxa belonging to 27 genera were identified, including 11 species of unambiguous eukaryotes, 10 species of possible eukaryotes or prokaryotes and 28 species of probable bacteria. This assemblage is more diverse than previously reported but includes taxa reported in coeval worldwide assemblages. It is characterized by abundant sphaeromorphs, filamentous colonial aggregates and filamentous forms, as well as a relatively low diversity of acanthomorphs including the Late Mesoproterozoic and Early Neoproterozoic index fossil – Trachyhystrichosphaera aimika – reported for the first time in Central Africa. This species co-occurs with other taxa also reported for the first time in Africa: Trachyhystrichosphaera botula , Jacutianema solubila , cf. Tappania sp., Valeria elongata and numerous other taxa. Correlation with other geochronologically constrained successions that contain Trachyhystrichosphaera confirms T. aimika as promising index fossil to define the Late Mesoproterozoic–Early Neoproterozoic interval. The available biostratigraphic data enable to suggest a minimum Tonian age for the Mbuji-Mayi Supergroup. This age is consistent with the published and new geochronological data. Comparison with worldwide Proterozoic assemblages permits to define microfossil assemblages useful for biostratigraphy. This study significantly improves our understanding of the diversity of the Late Mesoproterozoic–Early Neoproterozoic biosphere, and in particular the diversification of early eukaryotes, preserved in the Democratic Republic of Congo rock record and more broadly in Africa where micropaleontological investigations are sparse. [ABSTRACT FROM AUTHOR]

The Twangiza–Namoya Gold Belt (TNGB) developed in a Proterozoic, variably composed sedimentary sequence that is associated with felsic and mafic igneous rocks. During polyphase tectono-metamorphic episodes the basement was exposed to repeated periods of heat influx that mobilised hydrothermal fluids and, at the Meso- to Neoproterozoic boundary, led to the formation of “G4” granites and pegmatites. The evolution of the TNGB hydrothermal system and its associated gold mineralisation are poorly understood. Here we investigate the tourmaline major elemental and the boron isotopic composition from one pegmatite and seven tourmaline-bearing hydrothermal veins sampled from the Kamituga and the Lugushwa gold deposits. Tourmalines from the hydrothermal veins are essentially dravitic with variable Ca contents. In contrast, the pegmatitic tourmaline is schorl–elbaite dominated and largely Ca-free. The boron isotopic signature of hydrothermal tourmaline from the Kamituga deposit (δ 11 B: −18.4‰ to −15.3‰) is distinctly lighter than that from Lugushwa (δ 11 B: −13.0‰ to −8.9‰). Pegmatitic tourmaline from Kamituga shows intermediate δ 11 B values between −14.2‰ and −13.1‰ with an average of −13.9‰. The pegmatitic fluid most likely had a δ 11 B composition in the range of −12.8‰. The isotopically significantly lighter boron in tourmaline from hydrothermal veins from Kamituga cannot have formed by fractionation from this pegmatitic source. Using the Rb–Sr mineral isochron method we have dated the Kamituga pegmatite as 981 ± 16 Ma, which correlates in time with the regional late-Kibaran “G4” granites. Tourmaline-bearing veins from Lugushwa are significantly younger and formed during the early stages of the Pan-African orogenic period (677 ± 17 Ma; Rb–Sr mineral data). Fluid inclusions in quartz that are texturally associated with hydrothermal tourmaline contain abundant saline brines and CO 2 -rich fluids that are commonly associated with CH 4 and in places with amorphous carbon and N 2 . We interpret these fluids as having been derived from the hosting sedimentary sequence. This model is in agreement with the observed tourmaline composition, which are most readily affiliated with a variably Ca- and Al-rich pelitic source. Our model based on the isotopic, age and mineral compositional evidence indicates a sedimentary source of fluids forming the tourmaline-bearing hydrothermal veins in Kamituga and Lugushwa. [ABSTRACT FROM AUTHOR]

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530-560 Ma) and Early-Late Ordovician (440-480 Ma). [ABSTRACT FROM AUTHOR]

Based on geochronological U-Pb studies, the age of Wrangel Island granitoids was estimated as Neoproterozoic (Cryogenian). Some granitoids contain zircons with inherited cores with an estimated age of 1010, 1170, 1200, and >2600 Ma, assuming the presence of ancient (Neoarchean-Mesoproterozoic) rocks in the Wrangel Island foundation and their involvement in partial melting under granitoid magma formation. [ABSTRACT FROM AUTHOR]

Structures and textures of the peloidal wackestones, as well as size, shape, and composition of peloidal grains, from the Mesoproterozoic (Middle Riphean) Sukhaya Tunguska carbonate platform in the Turukhansk Uplift (Siberia) are considered. It is shown that these grains formed in the course of diagenesis were closely associated with the microsparitic replacement and the formation of molar tooth (MT) structures. Diagenetic transformations of rocks were related to the activity of anaerobic microbial communities inside the buried carbonate silt layers. The microbial activity during diagenesis was governed by the carbonate sediment composition and conservation mechanism of the high-molecular organic matter of primary producers therein, since this organic matter was the nutritious substrate for the primary anaerobe communities. [ABSTRACT FROM AUTHOR]

The present work addresses the long-standing issues on the characterization aspect of the Proterozoic siliciclastic successions exposed in the central part of the Lesser Himalaya, restricted between the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). Geologic, sedimentologic, and petrographic study divides the Lesser Himalaya in two zones- northern Palaeo- Mesoproterozoic Inner Lesser Himalayan (ILH) and southern Neoproterozoic Outer Lesser Himalayan (OLH) zones. The major lithofacies recognized from the zones are - (i) coarse grained siliciclastic (CGS), (ii) interbedded medium and fine-grained siliciclastic (IMFS), (iii) argillite (ARG), and (iv) siliciclastic-argillite rhythmites (SAR). Amongst all these facies, the nearshore IMFS facies shows consistent presence in both OLH and ILH zones. From the facies distribution pattern, a northwest-southeasterly trending palaeo- shoreline has been envisaged. The CGS facies in the ILH hints towards an alluvial fan setting during 1.8 Ga rifting phase associated with penecontemporaneous basic magmatism. Compositionally, the siliciclastics of both the zones (ILH and OLH) are arenite and wacke types with minimal variation in their detrital proportions, derived from the early Proterozoic (between 2.4-1.6Ga) Aravalli-Delhi Supergroup provenance. Nearly matching types and content of detrital modes and the lithofacies pattern of the ILH and OLH siliciclastics probably conclude the derivation from the rising (nearby) Aravalli-Delhi orogen and deposition in a foreland like situation. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Damara Sequence (>1000 m thick) is composed of siliciclastic and carbonate rocks that crop out in the Damara Belt, Namibia. In Damaraland (including the Vrede, Bethanis, Austerlitz and Toekoms farms), these rocks were deformed and metamorphosed under greenschist facies (biotite zone) conditions during the Damara Orogeny. The stratigraphy and paleoenvironments of the Damara Sequence rocks are debated by the scientific community. We use field data, including detailed 1:25,000 geological mapping, elaboration of stratigraphic profiles and observation of preserved primary structures, textures and composition, to identify lithofacies and lithofacies associations, and to interpret lateral stratigraphic relations. The Nosib and Otavi/Swakop groups of the Damara Sequence in the study area are herein interpreted as gravitational deposits, including up to 150 m thick slide blocks, related to the opening of the Outjo Basin. Debrites of the Nabis Formation (Nosib Group) are basement derived, and were developed in alluvial fan setting. Siliciclastic, carbonate and mixed successions of the Otavi/Swakop groups reflect slope to basin deposits and can be grouped in two tectonic sequences. The lower sequence consists mainly of slope and proximal submarine fan successions; medium to distal fan deposits characterize the upper sequence. A U–Pb SHRIMP zircon dating of a dacite layer yielded a 757 ± 5 Ma age for the base of the lower sequence. In the study area these sequences are limited by an unconformity. This unconformity records basin margin onlap and was later partially reactivated as a thrust fault during Pan-African compression. The Chuos and Ghaub formations, considered by several authors as Sturtian and Marinoan glaciogenic deposits, are interpreted alternatively as gravitational subaqueous mass flows. Rudstone beds similar to those of the Ghaub Formation occur in the Berg Aukas and Karibib formations and reinforce this interpretation. Large blocks, locally containing stromatolites are interpreted as olistoliths and isolated blocks (pebble to boulder size), are interpreted as sieved from gravitational flows, leaving a non glacial interpretation for these formations as a distinct possibility. [ABSTRACT FROM AUTHOR]

The uraniferous, Proterozoic Otish Basin of the southeastern Superior Province hosts shallow-dipping sills and northeast to northwest-trending dykes of the Otish Gabbro suite and the north-trending Matoush dyke. Otish Gabbro sills comprise two distinct geochemical groups that reflect different degrees of crystal accumulation in a common olivine-tholeiitic parental magma. Group 1 Otish gabbros approach liquid compositions, whereas Group 2 gabbros are crystal cumulates of olivine + plagioclase + clinopyroxene. The high Nb/Yb, La/Yb, Th/Yb and low Zr/Nb ratios of the Otish Gabbro sills reflect trace element-enriched, mantle-derived parental liquids, modified by the addition of a crustal component. A north-trending Matoush dyke records a second magmatic event in the Otish Basin, genetically unrelated to the emplacement of the Otish Gabbro sills. In contrast to the gabbros, the strongly altered Matoush dyke has more fractionated trace element patterns and strong enrichments in light rare earth elements (LREE) that are indicative of a lamprophyric affinity. The Otish Gabbro sills are part of a large ca. 2.17 Ga magmatic event that also generated the extensive Biscotasing and Payne River dyke swarms in the south-central and northeastern Superior Province, respectively. The contemporaneous mafic dyke swarms experienced relatively minor crustal contamination and their high Nb/Zr and Nb/Yb ratios suggest derivation from fertile mantle sources. Furthermore, the low Ti/Yb ratios of the ca. 2.17 Ga magmas are consistent with melting at relatively shallow depths (<100 km), implying removal of the depleted Archean lithosphere from a large portion of the Superior Province and replacement by a more fertile Proterozoic mantle. The variable geochemical composition of the ca. 2.17 Ga mafic magmas indicates melting of a compositionally heterogeneous mantle and appears inconsistent with a common distal mantle plume source. A more suitable petrogenetic model for the ca. 2.17 Ga dyke swarms of the Superior Province is vertical rise of mantle-derived magmas in a rifting environment. [ABSTRACT FROM AUTHOR]

Basinal shales of the lower Mesoproterozoic Kaltasy Formation, sampled from three boreholes drilled into the southeastern East European Platform, Russia, contain abundant and moderately well preserved microfossils. 34 distinct entities have been identified, most assigned to simple sphaeromorphic or small filamentous taxa found widely and characterized by long stratigraphic ranges. Ornamented microfossils found in coastal successions of other lower Mesoproterozoic basins are absent, but large filamentous microfossils interpreted as possible benthic photosynthetic eukaryotes are recorded, drawing comparisons to relatively deep water shales in Siberia. In overall aspect, the Kaltasy microfossils are consistent with other broadly coeval assemblages, but they highlight the importance of environment, as well as age, in determining the distributions of remains that record the early diversification of marine eukaryotes. Rectia magna is described as a new species. [ABSTRACT FROM AUTHOR]

The Romanian South Carpathians foreland is located southwards of the East European Craton (EEC) and consists of three major tectonic units: the Scythian Platform, the North Dobrogean Orogen and the Euxinic Craton. The Euxinic Craton is represented in the Dobrogea area by two components: (i) Central Dobrogean Shield and (ii) East Moesia. In central-northern Dobrogea, the Trans European Suture Zone (TESZ), including the Teisseyre–Tornquist Zone, presumably strikes below the surface. Because the paleogeographic provenance of the above tectonic units was poorly documented until present, a systematic sampling for detrital zircon has been conducted in all tectonic units except for the Scythian Platform basement which is entirely covered by the Danube Delta. In all, this totals 26 samples and around 1833 analyzed zircon grains. The distribution of zircon grains dates along the time scale for all the sampled terranes suggests that the sampled components of the Euxinic Craton in Dobrogea and the constitutive terranes of the North Dobrogean Orogen have a peri-Gondwanan, peri-Amazonian provenance. Their ages are Neoproterozoic or younger. In addition, the Euxinic Craton can be depicted as a paleo-Amazonian crustal fragment that was marginally affected by the Avalonian–Cadomian orogenic events. Along with its margin, the Avalonian type terranes accreted or Ganderian type terranes have evolved. Primary relationships between the Amazonian derived terranes and EEC are obscured due to an intricate geologic history exhibited by the North Dobrogean Orogen involving Caledonian, Variscan and Cimmerian orogenic events, while the geologic history of the Scythian Platform is also poorly established. The general distribution diagram of U–Pb detrital zircon ages shows some major zircon sources, shared with those detected at a global scale. They are episodic and coincident along with the time scale, such as those at the end of Archean or those from the Pan-African orogens. Finally, among these discussed tectonic components, only the Scythian Platform can possibly have an EEC provenance. [ABSTRACT FROM AUTHOR]

Paleogeographic data indicate that the Neoproterozoic Rapitan Group accumulated in two spatially related, silled basins during the first mid-Neoproterozoic glacial episode (ca. 711 Ma), yet the controls on glaciation, reason for basin development, spatio-temporal relationships among stratigraphic units, and constraints on iron formation deposition and distribution in this region and globally at this time have not been adequately explained. The lack of a coherent understanding of the group’s regional stratigraphy has been a significant barrier to deciphering the geologic history of this critically important succession. Based on new stratigraphic data, and incorporating the results of a related geochemical study, the Rapitan Group accumulated in tectonically complex sub-basins of a young rift zone. The spatial and temporal dynamics of these sub-basins controlled the grain size, volume, and distribution of detrital sediment (predominantly turbidites and diamictites) deposited during two glacially influenced episodes with ice cover. During an interglacial episode, the rotated, exposed upper surfaces of newly displaced rift blocks directed drainage away from basin margins, precluding significant clastic sediment delivery, and subaqueous margins of outboard tilted fault blocks acted as sills that isolated local bottom water, allowing the accumulation of an entirely hydrogenous succession (jaspilitic and hematitic iron formation). Two strike-parallel normal faults that strongly controlled basin-margin development are prominent thrust faults in the present-day orogen. Syndepositional activity of strike-normal transfer faults segmented the basins along their length, as reflected in pronounced along-strike variations in the presence/absence, thickness, and composition of the three stratigraphic units (Mt. Berg, Sayunei, and Shezal formations), and in the presence or absence of iron formation (locally >100 m thick; top of Sayunei Formation). Differential subsidence in actively rifting basins was essential for deposition of Rapitan-type iron formation, but glaciation, although coeval with Rapitan Group accumulation, was not a strong control. The timing of the onset of glacially influenced sediment deposition in the closely spatially related Rapitan (∼711 Ma) and Mount Harper groups (∼717 Ma) in northwestern Laurentia differs by ∼6 m.y., arguing for diachronous Sturtian glaciation, both locally and globally. [ABSTRACT FROM AUTHOR]