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Study on the petrology, LA-ICP-MS zircon U-Pb dating and whole rock geochemistry of the trachyandesite in Wufeng area of Yanbian, Jilin Province in Northeast China, shows that the SiO2 content is 58.33%-61.36%, K2O content 1.61%-3.28% and total alkali content 6.77%-10%, belonging to the medium potassium-high potassium calc-alkaline series. The weighted mean age of magmatic zircon 206Pb/238U is 175.52±2.7 Ma, which is consistent with the subduction time of Early Jurassic paleo-Pacific plate. The trachyandesite is characterized by relative enrichment of LILEs(K, Ba and Rb) and strongly incompatible elements (Th and U), and depletion of HFSEs (Nb, Ta and Ti), with high LREE/HREE ratio, and enriched LREEs and depleted HREEs, indicating that the magma may originate from the partial melting of mantle and its source area is metasomatized by the subducted shell source materials. From the above, it is believed that the trachyandesite was formed in the active continental margin of oceanic subduction system and was the magmatic response to the subduction of Early Jurassic paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

Granitoids, an important part of the continental crust, are the direct and indelible evidence of the accretion and reworking process of the continental crust in different tectonic settings. Therefore, these early geodynamic mechanisms can be roughly reconstructed by analysing the geochemical characteristics of granitoids. In this study, the Permian–Triassic Granitoids widely distributed in the East Kunlun Orogenic Belt (EKOB) are studied to determine their chronology and petrogenesis to restrict their source regions and establish the significance of their geodynamic and crustal growth. Zircon inductively coupled plasma mass spectrometry (ICP–MS) dating results show that the crystallization ages of these granitoids are between 252–240 Ma, during the early and middle Triassic. These granitoids have high SiO2 and medium Na2O + K2O contents, plotting into the granite region, and calc-alkaline to high-K calc-alkaline series on the SiO2–Na2O + K2O and SiO2–K2O diagrams, respectively. All the samples have aluminium saturation index (A/CNK) values from 1.00 to 1.25 showing peraluminous characteristics. In terms of trace and rare earth elements, the samples are depleted in high field strength elements (HFSEs) such as Nb, Ta, P, and Ti and are enriched in large ion lithophile elements (LILEs) such as Rb, K, Ba, and U. Moreover, all granitoids show right-inclined rare earth element (REE) distribution patterns with weak to moderate Eu anomalies (δEu = 0.50–0.93). In addition, the samples have low P2O5 contents, 10,000*Ga/Al ratios (1.65–2.39), and Zr + Nb + Ce + Y (212–262 ppm) displaying affinities to I-type granite. Combined with their εHf (t) values of −2.31 to 4.44, with 1.0–1.4 Ga two stage model ages (TDM2), they are considered to be the products of the reworking of the Mesoproterozoic lower crust; therefore, they are not related to the continental net growth in the EKOB. On the tectonic discrimination diagrams of granitoids, all the samples fall into the area of volcanic arc granite (VAG) and continental margin arc; this suggests that the Paleo-Tethys Ocean slab was still in the subduction stage during 252–240 Ma, and the continental collision had not yet started. Based on the plenty of magmatic rock dating results published previously, it appears there are two magmatic peak periods (260–240 Ma and 230–210 Ma) and one magmatic intermittence period (240–230 Ma) during the late Permian to Triassic of the EKOB, corresponding to three geodynamic evolution stages of subduction, post-collision extension, and syn-collision, respectively. [ABSTRACT FROM AUTHOR]

New geochemical and geochronological data lead to a fuller understanding of different aspects of the Rajmahal flood basalt province. Earlier work was on surface Rajmahal flows as well as from the nearby Bengal Traps. Samples from a ~300 m borehole near the northwestern corner of the Rajmahal lavas, analyzed herein, reveal the presence of carbonatitic (aillikitic) material. This small province, including the Bengal Traps, thus exhibits quartz and olivine tholeiites, andesitic basalts, alkali basalts as well as carbonatitic material. The Sylhet Traps, lying ~500 km to the east of the surface Rajmahal flows, show geochemical similarities, but were probably formed ~10 Myr after Rajmahal volcanism. Sr–Nd isotopic plots suggest that (a) not all lavas in the Rajmahal Traps were homogenized isotopically prior to eruption and (b) the Sylhet Traps show a much wider variety of isotopic values at eruption, possibly caused by alteration. Published and new 40Ar/39Ar ages for both the Rajmahal and other mafic rocks postulated to have been formed by the Kerguelen Hotspot in the Southern Indian Ocean, are critically examined; emphasis is placed on the freshness of the rocks dated. It is shown that a number of 'ages' for both the Rajmahal Province, and for rocks recovered from the Kerguelen Plateau area, should be rejected as invalid measures of the time of crystallization, as altered material was dated. For the surface Rajmahal material, valid ages span ~120 (quartz tholeiites) to ~116 Ma (andesitic basalts); alkali basalts and olivine tholeiites (Bengal Traps) as well as the aillikite were also formed within this time interval. The postulated genetic link to the Kerguelen Hotspot and basalts in the Southern Indian Ocean appears valid; in particular, a section of the southern Kerguelen Plateau (~58°S; 75°E) has strong geochemical and temporal links to the Rajmahal Traps. [ABSTRACT FROM AUTHOR]

The Guide Basin, located in the West Qinling orogen of central China, is a typical geothermal field. However, the granodiorites occurring in the Zhacanggou area have been poorly restricted. Herein, we report new isotopic compositions of whole‐rock major‐trace elements, zircon U–Pb, and Lu–Hf, on the host granodiorites and their microgranular enclaves (MEs) in the Zhacanggou area. Zircon U–Pb dating for the host rocks (porphyritic granodiorite and granodiorite) revealed ages of 230.5 ± 1.4 Ma and 229.0 ± 1.6 Ma, similar to those (230.5 ± 1.5 Ma and 231.4 ± 1.4 Ma) of the MEs. Zircon Hf isotope analyses for the granodiorite and MEs yielded εHf(t) values from −11.0 to −1.5 and from −7.8 to −0.7, with corresponding TDM2 ages of 1,355–1,959 and 1,304–1,755, respectively. The MEs possess plagioclase phenocrysts, acicular apatites, and chilled borders. They were characterized by relatively low SiO2 (58.39–65.24 wt%) and high Cr, Ni, and MgO contents, leading to relatively high Mg# values (44–59) and low Sr/Y (14.23–19.84) ratios. The host granodioritic rocks show similar adakitic geochemical characteristics with SiO2 = 64.23–68.57 wt%, MgO = 1.29–2.22 wt%, Mg# = 48–53, Sr = 340–596 ppm, Y = 11.3–13.7 ppm, Sr/Y = 28.39–47.89, and no obvious Eu anomalies. These geochemical and isotopic data, combined with the mineralogical features, indicate that the granodiorites in the Zhacanggou area were produced by the mixing of high‐Mg# melts originating from an enriched lithospheric mantle source, and felsic magmas deriving by the partial melting of the normal lower continental crust with the tectonic regime transition from a syn‐collision to a post‐collision setting. [ABSTRACT FROM AUTHOR]

The eastern flank of the Talesh range of western Alborz, northwestern Iran, exposes volumetrically significant Late Cretaceous volcanic and volcaniclastic associations as well as cross-cutting dykes and minor subvolcanic equivalents. The volcanic units appear as massive to pillowed flows interlayered with volcaniclastic beds and subordinate limestones. We present new field data, zircon U Pb ages, bulk-rock major and trace element geochemistry, and Sr Nd isotopes on the igneous rock suite. Geochemical data show that the lava units and dykes are mainly basaltic in composition with minor basaltic andesites and trachytes. They form two contrasting series of arc-related calc-alkaline (Group 1) and OIB-like (Group 2) rocks. The zircon U Pb age of Group 1 rocks is 95.6 ± 1.8 Ma (2σ), whereas for Group 2 rocks, cross-cutting relationships and stratigraphic constraints indicate a somewhat younger age. Group 1 rocks have 87Sr/86Sr (i) varying from 0.7038 to 0.7070 and ε Nd (t) values ranging from +0.5 to +5.0. Group 2 rocks have 87Sr/86Sr (i) = 0.7040–0.7065 and ε Nd (t) from −3.0 to +3.7. Trace element and isotopic modelling shows that the primitive Group 1 magmas were derived from a metasomatized mantle source enriched by the addition of ∼6–10 % sediment melt component, while Group 2 rocks are consistent with melts of an asthenospheric mantle source enriched by ∼1–5 % EMII component. Trace element modelling indicates that Group 1 rocks formed from ∼6–13 % partial melts of a spinel-garnet lherzolite with garnet:spinel ratios of 45:55 to 10:90, whereas primitive Group 2 melts were generated through ∼1–3 % partial melting of a garnet-spinel-lherzolite with garnet:spinel ratios of 90:10 to 80:20. We propose a geodynamic model in which a north-dipping Neotethyan slab beneath the western Alborz (Talesh area) continental margin produced arc assemblages of Group 1 rocks and subsequent slab rollback led to the upwelling of an asthenospheric mantle to generate OIB-like magmatism of Group 2 rocks in an extensional intra-arc to back-arc setting. [Display omitted] • The Talesh magmatic rocks are dominated by basaltic lavas. • They were emplaced during the Late Cretaceous. • The magmatic rocks include both arc-type calc-alkaline and OIB-like alkaline rocks. • The mantle source of arc-type rocks was enriched by slab-derived sediment melts. • Slab rollback triggered asthenospheric melting and formation of OIB-like magmas. [ABSTRACT FROM AUTHOR]

The Tianshui-Sunjiaxia area is located in the connection zone of West Qinling Orogen and North Qilian Orogen, which could provide great insights into the amalgamation processes between the northern and southern blocks of China. Three subduction- and rift-related rocks gneissic granite from North Qilian arc-interarc belt (NQAI) granite and metabasalt from North Qinling back-arc basin (NQBA) are distinguished across the connection zone. The gneissic granite was generated by melts from older crustal materials of Longshan Group with the addition of a relatively juvenile basaltic source from the lower crust during the collision process. The Liwanxincun metabasalt reflects the mixing of the partial melting of the shallow asthenospheric mantle and the metasomatized mantle in a back-arc extension setting. The LA-ICP-MS zircon U-Pb dating of gneissic granite (068, 069) yields crystallization ages of 457.0 ± 1.6 Ma and 445.9 ± 2.1 Ma. The study area is divided into six tectonic units in Early Paleozoic time involving NQAI (Yanjiadian-Xinjie) continental arc, interarc rift basin (Maojiamo-Xiwali), continental arc (Chenjiahe-Wangjiacha); NQBA back-arc rift basin (Huluhe-Hongtubao), island arc and ophiolitic melange belt (North Qinling-Shangdan). A tectonic model is proposed in which the NQAI continental arc (Yanjiadian-Xinjie) might represent the early period of subduction of North Qilian Ocean (NQO) and the interarc rift is the product of the extension triggered by southward subduction of NQO. The ongoing subduction of NQO then leads to the formation of Chenjiahe-Wangjiacha continental arc, as well as the Hongtubao back-arc spreading ridge in NQBA back-arc basin (Huluhe). The tectonic evolution of the connection zone is closely associated with the closure of the North Qilian Ocean and North Qinling-Shangdan Ocean in the context of the convergence process of micro-continental blocks, including North China block, Longshan group and North Qinling Terrane. [ABSTRACT FROM AUTHOR]

Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene "Lake Adobe" of limited aerial extent and a laterally extensive middle Eocene "Lake Elko," are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ18O, δ13C and 87Sr/86Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of 12C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of 87Sr/86Sr in the upper Elko Formation reflects an increasing aerial extent of low‐87Sr/86Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate. Plain Language Summary: Large lakes formed in the region that is now northeast Nevada between 50 and 40 million years ago. This region was at high elevation and undergoing a transition from compressional to extensional plate tectonic forces at that time. The geologic record shows that lakes formation started locally at 49.5 million years ago with a short‐lived freshwater Lake Adobe. Later, approximately 43.5 million years ago, a broad Lake Elko formed and then became increasingly evaporative over the next 3 million years, likely due to removal of the shallowly subducting Farallon slab from beneath the region. Key Points: Lakes formed in the hinterland of the North American Cordillera during the Eocene during a transition from contraction to extensionLake Elko expanded and became increasingly evaporative across 3.5 Ma from 43.5–40.5 Ma, coincident with an episode of explosive siliceous magmatismLake formation and drainage isolation likely records the progressive removal of the flattened Farallon slab from beneath the Cordillera [ABSTRACT FROM AUTHOR]

We present results of geochemical and Sr-Nd isotope studies of rocks and of local dating and determination of the Lu-Hf isotope composition of zircons from late Vendian-early Cambrian and Cambrian-Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the eNd values (6.9 to 0.5) of rocks and the eHf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian-early Paleozoic intrusive associations formed at the island arc and accretion-collision stages. Late Vendian (572-562 Ma, Kopto and Buren massifs) and early Cambrian (522- 518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian-early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro-monzodiorite-granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion-collision stage. Island arc complexes were the main source of Cambrian-Ordovician accretion-collision calc-alkalic plagiogranitoids (500-450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva-Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren' massif (450 Ma), marking the completion of accretion- collision processes in this region. The isotope parameters of the Vendian-early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion-collision processes. [ABSTRACT FROM AUTHOR]

The Bangong–Nujiang suture zone (BNSZ) is the remnants of the Meso-Tethyan main oceanic basin in the Tibetan plateau. It is noted that several microcontinents, e.g., Amdo and Jiayuqiao, occur within the Meso-Tethyan ocean basin. These microcontinents were strongly reworked associated with the subduction and closure of this ancient ocean, thus they play a key role in understanding the evolution of this ocean. In this contribution, we report newly documented Early Jurassic gabbros (ca. 182–175 Ma) from the Amdo microcontinent in the middle segment of the BNSZ. The gabbros can be subdivided into two groups, i.e., OIB (ocean island basalt) and MORB (mid-ocean ridge basalt) groups. The OIB-group gabbros are characterized by highly enriched Nd Hf isotopic characteristics (zircon ε Hf (t) = −11.7 to −8.9; whole-rock ε Nd (t) = −11.0 to −10.1). They are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (> 85 km) and subsequent continental crustal inputting. The MORB-group gabbros belong to the tholeiitic series and have similar compositions to those of MORB. They were most likely derived from a depleted mantle source (zircon ε Hf (t) = +3.7 to +16.5; whole-rock ε Nd (t) = +4.9 to +8.1) in a relatively shallow depth (< 85 km). Integrating previous studies with the data presented in this contribution, we suggest that the generation of coeval OIB- and MORB-like gabbros are related to lithospheric delamination after the Qiangtang-Amdo collision and the closure of the Bangong-Nujiang ocean basin. This result will give new constraint on the evolution of the Meso-Tethyan Ocean. • Early Jurassic gabbroic dikes were newly documented in the Amdo microcontinent, central Tibet. • Gabbroic rocks display different geochemical compositions with OIB and MORB features. • Zircon U-Pb dating on gabbroic rocks yielded nearly uniformed ages of 182–175 Ma. • Gabbroic rocks were formed by the lithospheric delamination after the Qiangtang-Amdo collision. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ediacaran-Cambrian series of Cap de Creus studied through Nd isotopic sources. • Eastern Pyrenees series (Cap de Creus) show Nd model ages ranging 1.4–1.6 Ga. • Coeval series from different Iberian terranes show contrasted Nd isotopic sources. • These data suggest different original locations along the Gondwana margin. • Isotopic tracers are a useful tool to check the provenance in paleoreconstructions. The North African margin of Gondwana formed by subduction and accretion during Ediacaran to Cambrian times. Sections of this active margin were incorporated during the late Paleozoic to the Variscan and Appalachian orogens. In the Variscan realm, these sections define in some regions a Cadomian basement comprising thick metasedimentary series associated with coeval subduction-related magmatic rock dated in the range 650–500 Ma. In the Cap de Creus Massif (Eastern Pyrenean domain), the Cadomian metasedimentary rocks formed from sediments derived from mixed felsic and intermediate/mafic sources, intruded by granitoid and minor mafic igneous rocks. These series were formed in a peri -Gondwanan volcanic arc setting, like other Cadomian series described in the SW of the Iberian Massif, such as Serie Negra Group of the Ossa Morena Complex. However, the Nd model ages of the Cap de Creus metasedimentary samples range from 1431 to 1620 Ma and are younger than those of their SW Iberia equivalents. This result can be interpreted as an indication of an eastern location in the Ediacaran - Cambrian peri -Gondwanan magmatic arc. Furthermore, the Cadomian series in the Central Iberian Zone have even younger Nd model ages than those of SW and NE Iberia. We present two end-member paleogeographic models consistent with reconstruction of the initial location of the Cap de Creus Massif and its relations with other domains (Ossa Morena Complex and Central Iberian Zone) in this sector of the North African Gondwanan margin. [ABSTRACT FROM AUTHOR]

• A continuum of Paleoproterozoic heritage, seen in Nd model ages, from N to S in the Bushmanland Domain (BD), with the Buffels River shear zone simply a regional discontinuity. • The southern BD (SBD) has a greater juvenile component than the northern BD (NBD), becoming more dominant post-1.10 Ga. • Orthogneisses derived from crustal magmas with subduction-influenced signatures with mixed Meso- to Paleoproterozoic model ages. The SBD Koperberg Suite derived from low degrees of melting of enriched SCLM, but at deeper levels than those of the NBD. • The paragneisses derived from largely reworked, mixed Meso- to Paleoproterozoic felsic to intermediate sources. • Paragneisses derived from largely reworked, mixed Meso- to Paleoproterozoic felsic to intermediate sources. • SBD Koperberg Suite derived from low degrees of melting of enriched SCLM, but at somewhat deeper levels than NBD Koperberg magmas. • D 2 deformation constrained to ~ 1.11-1.09 Ga, with peak M 3 metamorphism at ~ 1.05-1.02 Ga. The timing of tectonomagmatic events and petrogenesis of ortho- and paragneisses within the high-grade southern Bushmanland Domain (SBD) of the Namaqua Metamorphic Province is, largely, poorly constrained. U-Pb age dating and whole rock lithogeochemistry of the rocks of the upper granulite facies Kliprand dome provide constraints with regards to the evolution of the SBD during the 1.2–1.0 Ga Namaquan Orogeny. The SBD has a greater juvenile component, particularly added between ∼1.1 and 1.02 Ga, compared to the northern Bushmanland Domain (NBD). This is reflected in marginally younger T DM model ages (1.60–1.95 Ga) than for the NBD (∼1.9–2.4 Ga), and higher ε Nd(t) values for magmatic rocks, particularly for the late- to post-tectonic Spektakel Suite (SS), concentrated in the SBD. The ∼1.06 Ga Klein Lieslap Charnockite of the SS was derived from reworking of radiogenic Paleoproterozoic arc crust, but has a large juvenile depleted component compared to that of the ∼1.19 Ga Grootberg Gneiss of the syn-tectonic Little Namaqualand Suite. SBD mafic magmas were derived from low degrees of partial melting of subduction-influenced enriched subcontinental lithospheric mantle. The pre-tectonic ∼1.17 Ga Oorkraal Suite shows limited crustal contamination of mantle-derived melts. The post-tectonic ∼1.05 Ga Koperberg Suite was derived from deeper mantle melting than that in the NBD, but shows less crustal contamination. The supracrustal Kamiesberg Group was derived from mixed, reworked Meso- to Paleoproterozoic crust that showed a more dominant depleted component compared to that in the NBD. Incongruent anatexis of the paragneisses gave rise to crosscutting garnetiferous quartzofeldspathic segregations, notably the strongly peraluminous ∼1.07 Ga Ibequas Leucogranite. D 2 deformation, at ∼1.11–1.09 Ga, resulted in penetrative foliation. D 3 deformation, related to the development of a sub-vertical foliation in the limbs of F 2 folds, is constrained to ∼1.07–1.06 Ga, between emplacement of the Ibequas Leucogranite and the Koperberg Suite. Peak granulite facies M 3 metamorphism occurred at ∼1.05–1.02 Ga. These age constraints are similar to those of the NBD reflecting a synchronous tectonomagmatic and kinematic history. The SBD represents a deeper crustal level with a greater mantle influence than the NBD. [ABSTRACT FROM AUTHOR]

This article presents new zircon U-Pb geochronology, Hf isotopic, and whole-rock major- and trace-element geochemical data that provide insights into the petrogenesis and tectonic history of the Riwanchaka granodiorite porphyries of Central Qiangtang, Tibet. Zircon U-Pb ages of 236-230 Ma indicate an early Late Triassic age of emplacement of the porphyries, and zircon Hf isotopic data yield εHf(t) values of - 7.0 to - 1.5 and ancient zircon Hf crustal model ages (TDMC) of 1524-1220 Ma. The granodiorite porphyries are characterized by low K2O contents, high Mg# values, and relatively high Cr and Ni contents. They are classified as I-type calc-alkaline granite and are considered to have formed through the anatexis of ancient mafic crustal rocks with contributions from mantle-derived components. The geochemistry and isotopic compositions of all samples are similar to those of magmatic rocks that originated in the South Qiangtang crust. However, field observations indicate that the pluton intrudes the North Qiangtang crust, and we propose that the granodiorite porphyries were derived by partial melting of subducted continental crust of the South Qiangtang terrane. These new data have been integrated with data from previous studies to construct a new model of slab rollback during northward subduction of the Southern Qiangtang continental crust at ca. 245-226 Ma, thereby improving our understanding of magmatic processes involved in continental subduction in collision settings. [ABSTRACT FROM AUTHOR]

The Pliocene-Quaternary volcanics in NE Turkey are mainly hornblende-phyric trachyandesites having a narrow range of SiO2 from 61.88 to 63.00 wt.% and exhibiting adakitic signatures with their Na2O (3.67-4.27 wt.%), Al2O3 (16.19-16.80 wt.%), Y (14.1-16.5 ppm) contents and K2O/Na2O (0.87-1.12), Sr/Y (44.24-54.90), and La/Yb (36.80-43.88) ratios. Plagioclases as the main mineral phases show a wide range of compositions, and weak normal and reverse zoning. Hornblendes are generally edenite and pargasite (Mg#: 0.39-0.74). Clinopyroxenes are augite (Mg#: 0.58-0.76). Biotites have Mg# ranging from 0.45 to 0.66. The textural and compositional variations indicate disequilibrium crystallization possibly arising from magma mixing. The U-Pb zircon dating of the adakitic volcanics yielded 3.4-1.9 Ma. The studied rocks display moderate light rare earth element /heavy rare earth element ratios and enrichment in the lithophile element and depletion in high field strength element, implying that the parental magmas were derived from mantle sources previously enriched by slab-derived fluids and/or subducted sediments. The crystallization temperature and pressure estimations based on the clinopyroxene thermobarometry range from 1144 to 1186°C and from 3.92 to 7.97 kbar, respectively. Hornblende thermobarometry, oxygen fugacity, and hygrometer calculations yielded results as 908-993°C at a pressure of 2.87-5.22 kbar, water content of 4.4-8.4 wt.%, and relative oxygen fugacity (ΔNNO log units) of −0.6 to 0.9, respectively. Biotite thermobarometry suggests relatively higher oxygen fugacity conditions (10-13.33 to 10-17.60) at temperatures of 676-819°C and at pressures from 1.15 to 1.76 kbar. In the light of the obtained data and modelling, it can be concluded that the magmas of the adakitic volcanics were derived from enriched mantle source through relatively higher partial melting and experienced magma mixing with melts at the crustal level. Additionally, the fractional crystallization and assimilation-fractional crystallization processes may have played an important role during the evolution of the studied volcanics. [ABSTRACT FROM AUTHOR]

Abstract Large Igneous Provinces mark important and consequential events that span almost the entirety of the Earth's history. These punctuated geologically significant events that include earth processes from the core to the atmosphere have been linked to climate changes, mass extinctions, and even large-scale tectonic shifts. To begin to understand the role LIPs play in these important moments across Earth's lifespan, a thorough understanding of how they form in the first place is imperative. The ∼180 Ma Karoo Continental Flood Basalt (CFB) Province has been heavily studied with one of the largest geochemical and geochronological databases available for a CFB; providing an excellent avenue to study in detail the petrogenesis of these large collections of magma. However, despite this large dataset, the origin of the Karoo CFBs is still debated, with many different and conflicting models proposed. Disparities in these models can often be traced down to the degree of influence a plume and/or the sub-continental lithospheric mantle (SCLM) had during the genesis of this magmatic province. Nine sills intruded into the Western Cape Province of the Karoo Basin were investigated using 40Ar/39Ar geochronology, whole rock geochemistry, and Sr, Nd, Pb isotopes. Five of these sills are peculiar as they contain hydrated minerals (biotite and hornblende), an unusual feature for Karoo basaltic rocks. Biotite and/or hornblende separates yielded statistically indistinguishable 40Ar/39Ar ages as plagioclase separates; indicating that the biotite and hornblende found within these samples are primary features requiring that water was present in the magma during the crystallization of those sills. Major and trace element geochemistry place these rocks into the low-Ti suite of the Karoo CFBs. Trace elements indicate the involvement of fluids over sediment within the source region for these Karoo magmas. All samples from the Western Cape Province display enriched Sr–Nd–Pb initial values with large variations in elemental concentrations for a restricted range of isotopic ratios. Assimilation models indicate these enriched isotopic contents and observed trends are not the result of crustal contamination and are representative of the composition of the source. The trace element and isotopic geochemistry as well as the primary hydrated minerals indicate that water present in the system at the time of formation came directly from one of the mantle sources, supporting the hypothesis that the SCLM was metasomatized by fluids plausibly as a result of the Proterozoic Kibaran Orogeny and/or the Permian Cape Orogeny. This first direct evidence of water within the mantle sources of tholeiitic CFBs, until now only theorized and modeled, potentially changes the fundamentals in how we approach our considerations of CFB generation as a whole. Highlights • Primary hydrous minerals provide first evidence of hydrated material within the Karoo magmas. • The major, trace, and isotopic geochemistry of these hydrous dolerites indicate a hydrated source for the Karoo magmas. • These results support the hypothesis that the SCLM was metasomatized by fluids from ancient subduction. [ABSTRACT FROM AUTHOR]

The oldest rocks of the Rio Negro Province in the Amazonian Craton, have a complex evolutionary history and little known about them. In this paper are presented field, microtextural data, amphibole and plagioclase mineral chemistry, zircon U–Pb ages, and Sm–Nd isotope composition from Içana Domain basement rocks which belong to the Rio Negro Province. The studied area of the province is located in the extreme northwestern portion of Brazil (Amazonas State). The integration of the data obtained in this study allowed to propose the tectono-metamorphic evolution of Içana Domain in the Rio Negro Province. The basement rocks comprise orthogneiss and metagranite of the Cauaburi Complex and homogeneous/heterogeneous diatexite of the Taiuaçu–Cauera diatexite. The Cauaburi Complex rocks exhibit a predominantly porphyroclastic texture and has a mafic mineralogy constituted by biotite, amphibole, sphene, epidote, and allanite. On the other hand, Taiuaçu–Cauera diatexite varies from inequigranular to porphyroclastic, being constituted by biotite, muscovite, allanite, and epidote. Basement rocks of the Içana Domain record three tectonic–metamorphic events, which were associated to regional events. The M 1 /D 1 event was marked by the syn-tectonic emplacement of the Cauaburi Complex protolith rocks (1813 ± 19 Ma), and also by crystallization (1821 ± 14 Ma) and migmatization (1788 ± 11 Ma) of the Taiuaçu-Cauera diatexites in an orogenic event. The M 2 /D 2 event reached high temperatures, similar to the M 1 /D 1 event and was related to emplacement of S–type granites in the Içana Domain basement during the Içana Orogeny (ca. 1520 Ma). Furthermore, the lower temperature of the M 3 /D 3 event was related to intracontinental reworking of the K'Mudku event (ca. 1200 Ma). The Cauaburi Complex basement rocks shows epsilon Nd (t) values range from +1.53 to +0.13, which allow the identification of a crustal contribution to the magma source (likely Orosirian sialic crust). Geochemistry data indicate that these rocks have been generated from partial melting of amphibolitic rocks, and also with an important crustal component related to magmatic arc settings. All igneous (1967 ± 25 Ma) and metamorphic (1911 ± 15 Ma) zircons grains within the Cauaburi Complex basement rocks are compatible with those found in Tapajós-Parima/Ventuari–Tapajós Province, reinforcing the crustal contribution hypothesis. Geochemistry and mineralogical data indicate that Taiuaçu-Cauera diatexite was likely formed by the partial melting of the metagraywacke. Two age populations (1993 ± 33 and 1842 ± 9 Ma) from detrital zircon of the homogeneous diatexite, suggest that the sediments were derived from different rocks of the Tapajós-Parima/Ventuari–Tapajós Province. The age of the Taiuaçu-Cauera diatexite is probably between 1842 ± 9 Ma (youngest detrital zircon) and 1788 ± 11 Ma (anatectic zircon). The youngest detrital zircon (1842 ± 9 Ma) and anatectic zircons (1788 ± 11 Ma), indicates that the protolith of Taiuaçu-Cauera diatexite is not a part of the Tunuí basin (Tunuí Group). [ABSTRACT FROM AUTHOR]

The Sierra Nevada de Santa Marta in NW Colombia is an isolated massif at the northernmost end of the Andes chain near the boundary with the Caribbean plate. Previous geologic mapping and K-Ar dating have shown that Jurassic plutonic and volcanic units make up a large part of the Santa Marta Massif (SMM). These rocks have been considered to be part of a Jurassic magmatic arc extending from NW Colombia to northern Chile, but without any geochemical basis for comparison. This paper reports on a geochemical and Sr-Nd-Pb isotope study of the Jurassic rocks in the SMM and provides 12 new U-Pb zircon ages from in-situ laser ICP-MS dating. The plutonic and volcanic units span a range from 45 to 78 wt.% SiO 2 , with a dominance of intermediate to felsic compositions with SiO 2 > 57 wt.%. They classify as calc-alkaline, medium to high-K, metaluminous rocks with trace-element features typical for arc-derived magma series. In terms of their major and trace-element compositions, the SMM Jurassic units overlap with contemporary plutonic and volcanic rocks from other regions of the Central and Eastern Cordilleras of Colombia, and confirm an arc affinity. The new U-Pb ages range from 176 ± 1 Ma to 192 ± 2 Ma (n = 12), with most between 180 and 188 Ma (n = 7). The initial Sr isotope ratios (at 180 Ma) are between 0.7012 and 0.7071 (n = 29), with 3 outliers attributed to mobilization of Rb and/or Sr. Nearly all samples have negative εNd (180) values between −10.3 and 0.0 (n = 30), the two exceptions being only slightly positive (1.1 and 1.9). Measured Pb isotope ratios fall in a narrow range, with 206 Pb/ 204 Pb from 18.02 to 19.95, 207 Pb/ 204 Pb from 15.56 to 15.67 and 208 Pb/ 204 Pb from 37.76 to 39.04 (n = 28). In the regional context of previous studies, these results confirm early Jurassic ages and an arc affinity for the widespread magmatism exposed in the eastern and northeastern Colombian Andes. We also note patterns in the distribution and composition of magmas. The magmatic activity in the Central Cordillera tends to be younger than in the Eastern Cordillera and is spatially more restricted to the vicinity of regional fault systems. In terms of composition, Jurassic igneous rocks in the Eastern Cordillera have systematically lower εNd (180) values than those from the Central Cordillera, whereas the Pb isotope ratios overlap. We ascribe the Nd isotope variations to heterogeneity in the mantle source and/or degree of crustal contamination, whereas the Pb isotope ratios are crust-dominated and similar throughout the region. The spatio-temporal and compositional evolution of Jurassic magmatic rocks in the Northern Andes reflect the major plate kinematic readjustment between the Triassic and the Early Jurassic in the proto-Andean margin. [ABSTRACT FROM AUTHOR]

This paper presents new zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data for two granitic plutons and rhyolites of the Baiyingaolao Formation in the western Xing’an range (NE China). The two syenogranite granitic plutons yield identical zircon U-Pb age of 142 ± 1 Ma, and the Baiyingaolao rhyolites yield zircon U-Pb age of 138 ± 2 Ma. The granites contain some hornblendes, and show low Zr and Zr + Nb + Ce + Y contents, and low A/CNK (0.98-1.11), Mg# (6-55), and FeOT/MgO values. Rhyolite samples show similar geochemical characteristics with A/CNK of 0.99-1.10 and Mg# of 14-21. In combination with the high K2O contents (4.43-5.61 wt%) and negative correlations between P2O5 and SiO2, both the granites and rhyolites were classified as high-K calc-alkaline I-type granitoids. All samples give high zirconium saturation temperature of 794-964 °C with few initially inherited zircons, and belong to high-temperature I-type granitoids. They were generated by dehydration melting of biotite/muscovite from sub-alkaline meta-basalts in lower crust depth, leaving garnet, amphibole, and plagioclase as the major residual minerals. The syenogranites and rhyolites are likely formed in Mongol-Okhotsk oceanic subduction setting. Incorporating other lower crust-originated felsic rocks in Erguna and Xing’an massifs and Songliao basin, it is argued that lower crustal reworking is pronounced in NE China during Early Cretaceous. [ABSTRACT FROM AUTHOR]

The geochemistry, geochronology, and isotope geochemical systematics (Nd, Sr, Hf, and Pb) of the granitoids of the Pozdnestanovoy complex of the Dzhugdzhur-Stanovoy superterrane of the Central Asia fold belt were investigated. It was shown that their age is Mesozoic (142-138 Ma) rather than Early Precambrian, as was previously supposed. The main sources of parental melts for these granitoids were the Neoarchean and Paleoproterozoic rocks of the lower continental crust of the Dzhugdzhur-Stanovoy superterrane and the rocks of the Late Paleozoic-Early Mesozoic continental crust of the Amur microplate. They were formed at depths of >40 km and temperatures of 700-800°C, most likely through the melting of mafic feldspar granulites under the conditions of aqueous fluid infiltration without any significant contribution from a juvenile heat source. The granitoids of the Pozdnestanovoy complex were emplaced during the closure of the eastern segment of the Mongolia-Okhotsk Ocean owing to the collision of the Siberian and Sino-Korean continents. [ABSTRACT FROM AUTHOR]

The results of geochronological (U-Pb), isotope-geochemical (Lu-Hf), and geochemical (REEs) studies of young (MZ, KZ) and xenogenic (AR, PR) zircons from magmatic rocks of the Central Arctic rises of the Arctic Ocean (AO) and the crest zone of the Mid-Atlantic Ridge (MAR) are presented. The data obtained show that the depleted mantle could be a source of young (KZ) zircons of the MAR, whereas young (MZ) zircons of the MAR and all xenogenic (AR, PR) zircons of the AO and MAR are from crustal rocks of the continental lithosphere. [ABSTRACT FROM AUTHOR]

The paper reports first geological, chemical, mineralogical, Sr-Nd chemical-isotope, and geochronological data on the gabbroid massif discovered on the Hobbs coast in the Cape Burks area, West Antarctica. The area is made up of compositionally diverse gabbroids that are intersected by thin vein and dike bodies of mafic, intermediate, and fesic composition. The gabbroids are represented by olivine and olivinefree gabbros and gabbronorites, with sharply subordinate troctolites, gabbro-anorthosites, and anorthosites. The U-Pb SHRIMP-II zircon age of the gabbroids and vein rocks was estimated at 100 ± 1 Ma. The gabbroids were supposedly emplaced in the upper crust in tectonically active conditions. The thickness of the pluton is no less than 2.5-3 km. The rocks were crystallized from a highly fractionated melt. Their composition was mainly determined by accumulation and fractional crystallization. The origin of vein felsic rocks was likely related to an evolved residual liquid. The igneous complex was formed in a within-plate geodynamic setting, and its primary melts were derived from a weakly LILE enriched lithospheric mantle. [ABSTRACT FROM AUTHOR]

This paper reports zircon U–Pb geochronology, whole-rock chemistry and Sr–Nd–Hf isotopic data of intermediate-acid intrusive rocks from the Gerlache Strait area, Antarctic Peninsula. These intrusive rocks can be divided into three groups. Group #1 has an emplacement age of ca. 117 Ma, and is further subdivided according to K 2 O contents into High-K (HKS) and Low-K (LKS) Subgroups. The HKS monzogranites are weakly peraluminous, and yield ε Hf ( t ) and ε Nd ( t ) values of +0.9 to +2.3 and +0.1 to +2.3, respectively, with corresponding model Nd ages (T DM Nd ) of 766–471 Ma, indicating that they were derived from the partial melting of dominantly early Neoproterozoic to early Paleozoic juvenile basaltic materials with minor involvement of sediments. In contrast, the LKS granodioritic magma was derived from partial melting of early Paleozoic juvenile basaltic materials with ε Hf ( t ) and ε Nd ( t ) values of +10.3 to +12.5 and +5.4 to +5.5, respectively, and corresponding T DM Nd ages of 548–500 Ma. Group #2 consists of granite porphyry that formed at ca. 82 Ma and is characterized by positive values of ε Hf ( t ) (+5.1 to +8.1) and ε Nd ( t ) (+1.9 to +3.1) with weakly peraluminous chemistry and corresponding T DM Nd ages of 712–548 Ma, suggesting that it was generated from partial melting of dominantly Neoproterozoic juvenile basaltic materials with minor involvement of sediments. Group #3 comprises dioritic–granodioritic rocks that were emplaced at ca. 62–54 Ma. These rocks have Mg# values of 39–46, and ε Hf ( t ) and ε Nd ( t ) values of +6.7 to +12.2 and +3.6 to +7.2, respectively, with corresponding T DM Nd ages of 529–307 Ma, indicating that they were derived from partial melting of Paleozoic juvenile basaltic materials with minor injection of mantle materials. Spatially, the lithology of these plutons roughly changes from acid to intermediate from east to west, concomitant with increasing ε Hf ( t ) and ε Nd ( t ) values and decreasing initial 87 Sr/ 86 Sr ratios and T DM Nd ages. This trend reflects changes in source characteristics from a Gondwana affinity to a juvenile crust affinity. Overall, the southeast-to-northwest migration of magmatism in the northern Antarctic Peninsula since the Mesozoic may have been related to retreating subduction of the paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

The geochemistry and detrital zircon geochronology of the Quemoco sandstones from the Woruo Mountain area, North Qiangtang Basin, are analysed to discriminate their provenance and tectonic setting. The sandstones were classified as litharenite according to their geochemical data. The index of chemical variability and SiO2/Al2O3 ratio values suggest that the compositional maturity and recycling are weak to moderate. The weathering indices, such as chemical index of alteration, plagioclase index of alteration, and chemical index of weathering, and A–CN–K (Al2O3–(CaO* + Na2O)–K2O) ternary plot reflect that the source areas have undergone a weak degree of weathering. The TiO2 versus Zr, La/Th versus Hf, and Co/Th versus La/Sc bivariate diagrams and multimajor elements discrimination diagram of Quemoco sandstones indicate that the provenances were primarily derived from felsic igneous rocks and may also have less intermediate igneous source rocks. The paleocurrent data and low composition and textural maturity of the Quemoco sandstones suggested that the provenances are mainly from the southwest of the study area and with proximal source features. The U–Pb ages studied here could be divided into 8 groups: 206–219 Ma, 268–283 Ma, 446–630 Ma, 700–970 Ma, 1006–1346 Ma, 1457–1843 Ma, 2087–2214 Ma, and 2331–2587 Ma. The provenances are mainly from the Central Uplift Belt and surrounding of the study area according to the detrital zircon ages and rare earth element parameters, which are composed of Late Triassic tuff, Permian basalt, Early Ordovician granite, Cambrian granite gneiss, and Precambrian crystalline basement. Two multidimensional tectonic discrimination diagrams based on major elements show a complicated tectonic setting composed of a collision, rift, and minor arc setting for the Quemoco sandstones, which is consistent with the nature of a rifted margin during the late Late Triassic. [ABSTRACT FROM AUTHOR]

Adakitic intrusions are supposed to have a close genetic and spatial relationship to porphyry Cu deposits. However, the genesis of adakitic intrusions is still under dispute. Here, we describe newly discovered intrusive complex rocks, which are composed of ore-bearing, layered magnetite-bearing gabbroic and adakitic rocks in Jiamate, East Junggar, NW China. These Jiamate Complex intrusions have diagnostic petrologic, geochronologic and geochemical signatures that indicate they were all generated from the same oxidized precursor magma source. Additionally, these layered rocks underwent the same fractional crystallization process as the ore-bearing adakitic rocks in the adjacent Kalaxiangar Porphyry Cu Belt (KPCB) in an oceanic island arc (OIA) setting. The rocks studied for this paper include layered magnetite-bearing gabbroic intrusive rocks that contain: (1) gradual contact changes between lithological units of mafic and intermediate rocks, (2) geochemical signatures that are the same as those found in oceanic island arc (OIA) rocks, (3) typical adakitic geochemistry, and (4) similar characteristics and apparent fractional crystallization relationships of ultra-basic to basic rocks to those in the nearby Beitashan Formation and to ore-bearing adakitic rocks in the KPCB. They also display similar zircon U-Pb and zircon Hf model ages. The Jiamate Complex intrusions contain intergrowths of magnetite and layered gabbro, and the intermediate-acidic intrusions of the Complex display typical adakitic affinities. Moreover, in conjunction with previously published geochronological and geochemistry data of the mafic rocks in the Beitashan Formation and in the KPCB area, additional data generated for the Jiamate Complex intrusions rocks indicate that they were formed from fractional crystallization processes. The Jiamate Complex intrusions most likely were derived from a metasomatized mantle wedge that was underplated at the root of the Saur oceanic island arc (Saur OIA). The ore-bearing adakitic intrusions in the KPCB and the adakitic Jiamate Complex intrusions were both probably generated from the same basaltic parental magmas through fractional crystallization. In addition, characteristics of the layered, magnetite-bearing, oxidized, basaltic Jiamate Complex intrusive rocks indicate that they are likely to be the parental arc magmas for the nearby porphyry Cu deposits. This conclusion is based on new interpretations of the regional and local geology, on interpretation of new geochemical analysis, new stable isotope analysis, new geothermobarometry, and new zircon age dating as well as other techniques and interpretations. [ABSTRACT FROM AUTHOR]

The Xiaoyao tungsten polymetallic skarn deposit in the eastern Jiangnan Massif of Yangtze Block is located at the contact between a granodiorite pluton and Sinian–Cambrian limestone. The intrusions in the tungsten-rich district comprise the Xiaoyao and other granodiorite plutons and granite porphyry dikes. The age determinations by LA–ICP–MS U–Pb dating of zircons indicate that the granodiorite formed at 149.4 ± 1.1 Ma, whereas the granite porphyry was emplaced at 133.2 ± 0.7 Ma. Re–Os dating of molybdenite from the skarn orebodies yielded a weighted average age of 148.7 ± 2.3 Ma (n = 5). These ages indicate that the tungsten mineralization is temporally related to the granodiorite. The granodiorites are metaluminous (A/CNK = 0.86–0.98) and in the high-K calc-alkaline series. They contain hornblende and have a negative correlation between P 2 O 5 and SiO 2 , indicating that they are typical I-type intrusions. The granite porphyries exhibit high alkali contents (Na 2 O + K 2 O = 7.97–9.53%), elevated FeO T /(FeO T + MgO) ratios (0.83–0.94), high concentrations of Zr, Nb, Ce, and Y, and high Zr saturation temperatures (average of 812 °C); thus, they are geochemically similar to A-type intrusions. The initial 87 Sr/ 86 Sr and ε Nd (t) values range respectively from 0.7074 to 0.7083 and from − 7.9 to − 1.3 for the granodiorite, and from 0.7008 to 0.7083 and from − 6.3 to − 4.7 for the granite porphyry. In addition, two-stage Nd model ages (T 2DM ) of 1.0–1.6 Ga for the granodiorite and 1.3–1.4 Ga for the granite porphyry indicate that the Proterozoic crustal rocks of the Shangxi Group could have contributed to the Xiaoyao magmas. The rhenium contents of the molybdenite grains vary from 32 to 136 ppm, suggesting that the molybdenum was derived mainly from a mixture of mantle and crustal sources. Based on the new geochemical data and regional geological investigations, we propose that the Late Jurassic mineralization-related I-type granodiorite was derived from the Neoproterozoic Shangxi metamorphic rocks with some additional input of mantle material. The magmatism was triggered by asthenospheric upwelling induced by slab tearing during oblique subduction of the paleo-Pacific plate beneath the South China Block. [ABSTRACT FROM AUTHOR]

The Matou Mo(-Cu) deposit, located in the Yangtze Valley Metallogenic Belt of central-eastern China, is a typical porphyry-type Mo deposit. The orebodies at the deposit are hosted by Matou porphyritic granodiorite, which is the largest intrusive in the area. Quartz vein-type and disseminated sulfide mineralization are well developed in the porphyry and near its contact with Silurian sandstone. Crosscutting relationships indicate that porphyritic granodiorite is the oldest phase in the pluton, which is crosscut by a porphyritic diorite containing traces of chalcopyrite, and later dolerite dykes. These phases have U-Pb zircon dates of 147 ± 3, 140 ± 1 and 135 ± 1 Ma, which confirms the cross-cutting relationships observed in the field. A Re-Os molybdenite isochron age of 147 ± 4 Ma indicates that the porphyritic granodiorite is the source of the oldest Mo mineralization in the metallogenic belt and was formed during a change of the tectonic setting in the area, from an intracontinental orogeny to extensional tectonics. From 147 to 135 Ma, crust-mantle interaction played an important role in the formation of magmatic rocks at Matou. Systematic petrological and geochemistry investigations reveal that the three phases have a crust source with minor input from the mantle. Investigation of ore-forming fluid, H-O isotopes, S isotopes, and the Re content of molybdenite indicate that the ore-forming fluid and metals were derived from the lower crust. During the evolution of fluid from initial magmatic fluids (stage I) to ore-forming fluids (stage II), fluid boiling accompanied by the input of relatively cooler meteoric water led to the deposition of the Mo mineralization. [ABSTRACT FROM AUTHOR]

The Siah-Kamar porphyry Mo deposit, located in the western Alborz-Azarbayjan magmatic belt, is the first and largest Mo deposit in the Iran. This deposit is mainly hosted by an I-type, shoshonitic quartz monzonite to monzonite intrusion and also extends in the surrounding lower to middle Eocene volcanic rocks. The geochemical features of the Siah-Kamar intrusion show enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and significant negative anomalies of Nb, Ta and Ti analogues to the magmas derived from metasomatized sub-continental mantle. Porphyry molybdenum mineralization is associated with potassic, sericitic, argillic, and propylitic alteration zones. Mineralization occurs in disseminated form, in veins/veinlets and in hydrothermal breccias. The main ore minerals comprise molybdenite, chalcopyrite and bornite. The Microthermometric analyses at Siah-Kamar deposit showed that the halite-bearing inclusions contain high salinity (30.9–60.7 wt% NaCl eq.) with homogenization temperature ranging from 226 °C to 397 °C. The homogenization temperature of two phase liquid-rich inclusions range between 224 °C and 375 °C. The salinity of this type inclusions range from 0.6 to 7.5 wt% NaCl equivalent. The two-phase vapor-rich fluid inclusions homogenized at 270 °C to 397 °C. The salinity of this type fluid inclusions lie within the range of 0.6 to 4.24 wt% NaCl equivalent. Coexisting two phase V-rich and L-rich fluid inclusions in quartz associated with molybdenite provide evidence for boiling at 270 °C to 400 °C. The δ 18 O water values of quartz in the molybdenite-bearing veins vary from +2.16 to +4.05‰, suggesting a magmatic origin for the ore-forming fluids. Re-Os isotopic dating of molybdenite indicated a mineralization age of 41.9 ± 3.6 Ma. The Re concentration in molybdenite suggests incorporation of mantle derived melt with crustal materials. The late Eocene magmatism along the western Alborz-Azarbayjan magmatic zone resulted from the Neo-Tethys subduction beneath the Iranian plateau. The Siah-Kamar monzonitic intrusion hosting the Mo deposit, could be considered as an example among the late Eocene intrusions within the western Alborz-Azarbayjan magmatic zone for any further exploration in this zone. [ABSTRACT FROM AUTHOR]

Located in the eastern section of the Central Asian Orogenic Belt, the Jiawula Ag-Pb-Zn deposit is classified as a volcanic to subvolcanic related vein-type ore deposit. New U-Pb zircon geochronology, whole-rock geochemistry, mineral chemistry, and Sr-Nd isotope data are presented for the intrusions in the Jiawula deposit in order to evaluate the timing, petrogenetic type of the granitoid rocks, origin and evolution of magmatism, geodynamics, and to establish its relationship with lead-zinc mineralization. Zircon SHRIMP U-Pb analyses yield weighted mean ages of 150.1 ± 1.8 Ma for quartz porphyry, 148.8 ± 2.2 Ma for syenite porphyry, and 145.3 ± 1.9 Ma for monzonite porphyry, indicating a Late Jurassic (Yanshanian) magmatic event. An earlier magmatic event (Indosinian) occurred during the Late Permian to Early Triassic from ca. 254 Ma to ca. 247 Ma and is represented by granodiorite (254 ± 2 Ma), dacite porphyry (252.9 ± 4.8 Ma), and diorite porphyry (278 ± 4.1 Ma). Both the Indosinian and Yanshanian igneous rocks are classified as I-type granitoids. The late Jurassic intrusions are highly fractionated and characterized by negative anomalies of Eu, Sr, P, and Ti. The hypabyssal intrusions have initial 87 Sr/ 86 Sr values between 0.70458 and 0.70522, and ε Nd (t) values of −3.4 to −0.2, indicating relatively older crust in Jiawula among more juvenile crust in this area. Magma generation in Jiawula is linked to juvenile lower crustal and slightly enriched mantle sources. The ∼250 Ma magmatic episode in Jiawula might be related to the subduction of the Mongol-Okhotsk oceanic plate towards the south beneath the Erguna massif. The ∼150 Ma magmatic event occurred after the closure of the Mongol-Okhotsk Ocean followed by the change in subduction direction of the Paleo-Pacific plate. Varying temperature, stronger fractionation and higher oxygen fugacity related to the magmatic-hydrothermal transition caused Pb-Zn mineralization. [ABSTRACT FROM AUTHOR]

Granites, especially those that were decompressed as a result of processes of secondary transformations and tectonic movements, hold the greatest potential in the search for oil-bearing rocks in basements of sedimentary basins. Owing to the chemical dating of monazites from collision granites of the East Shebur area, the isochrone age of 274.5 ± 21 Ma was obtained. Zircons (SHRIMP-II) yield a concordant age of 277.5 ± 2 Ma (MSWD = 0.77). The Rb-Sr isochrone age of granites (247.4 ± 9.1 Ma) coincides with the emplacement time of the West Siberian rift system. This age records the process of intruding East Shebur granites into the upper crust level during riftogenesis and the extension of the study area. The younger age datings (231 Ma, 60 Ma (U-Pb); 220 Ma (K-Ar)) record the stages of tectonomagmatic activation, recorded in the basement of the Western Siberia oil-bearing areas. [ABSTRACT FROM AUTHOR]

This work presents the results of U-Pb isotope dating of zircons from granodiorites and plagiogranites of the Tallainskii gabbro-granodiorite-plagiogranite complex of the Karalon-Mamakan zone of the Baikal-Muya belt, ascribed to the Tallainskii pluton. The age datings obtained for granodiorite of the Eleninskii massif (605 ± 6 Ma) and plagiogranite of the Ust-Berezovo massif (609 ± 6 Ma) are in close agreement within the limits of error. Taking into account previously published data, the emplacement of the Tallainskii complex occurred within the age interval of 615-603 Ma in connection with postcollision extension. The 'island arc' geochemical characteristics of granodiorites and plagiogranites can be explained by magmatic differentiation and (or) participation in the formation of a melt source enriched in the suprasubduction component involved in petrogenesis during the preceding Neoproterozoic period. [ABSTRACT FROM AUTHOR]

Economic copper, nickel, and platinum-group element (PGE) deposits are associated with the Talnakh ultramafic-mafic intrusion of the Noril'sk Province in the northwestern part of the Siberian Platform. The results of the chemical Th-U-Pb dating of monazite from the ore-bearing melanotroctolites of the Talnakh intrusion are presented. The isotope-geochemical features of monazite are compared with zircon, sulfides, and baddeleyite. [ABSTRACT FROM AUTHOR]

Northern Hemisphere tropical paleoclimate records support significant changes associated with Dansgaard Oeschger (D/O) events and Heinrich stadials 1 to 6 during the last 64,000 years. However, few absolutely dated terrestrial records from the western Atlantic span the last six Heinrich stadials. Here we present geochemical results from three new stalagmites collected from a cave in the Bahamas which encompass Heinrich stadials 1 to 6. We build on a previous study of the δ 13 C and δ 18 O values of the calcite and δ 18 O value of fluid inclusions from a single stalagmite from the same cave spanning the last three Heinrich stadials. Absolute geochronometry using U-Th equilibrium series demonstrates that the stalagmites formed between 63.8 and 13.8 kyr BP. The δ 13 C and δ 18 O values of the calcite show higher values associated with Heinrich stadials 1–6, and lower values during the D/O interstadial events. The Sr/Ca ratios of the calcite are shown to be relatively invariant, while in two of the samples the Mg/Ca ratios track the δ 13 C values. Increases in the δ 18 O values across Heinrich stadials 1–6 are interpreted as being driven by lower temperatures. The two deeper occurring stalagmites demonstrate increased Mg/Ca ratios and δ 13 C values during Heinrich stadials 1 and 2 which are interpreted as a signal of reduced flow rates in the epikarst and increased water/rock interactions as a result of increased aridity which potentially occurred across all six Heinrich stadials. The observed reductions in mean annual temperature and amount of precipitation across Heinrich stadials are proposed to be driven by a reduction in sea surface temperatures in the North Atlantic and an expanded Bermuda High. During D/O interstadials, the Bahamas cave records likely indicate warmer and/or wetter climate; however the isotopic shifts are not as significant as the isotopic excursions associated with Heinrich stadials. [ABSTRACT FROM AUTHOR]

New geochronological, geochemical, and isotopic data are reported for the Capilla del Monte two-mica granite pluton in the northeastern Sierras de Córdoba. An Early Carboniferous age is established by a U-Pb zircon concordia (336 ± 3 Ma) and a Rb-Sr whole-rock isochron (337 ± 2 Ma). Zircon saturation geothermometry indicates relatively high temperatures (735-800 °C). The granites have high average SiO (74.2 %), NaO + KO (7.8 %), and high field-strength elements, high KO/NaO (1.7) and FeO/MgO ratios (5.1), with low CaO content (0.71 %). REE patterns with marked negative Eu anomalies (Eu/Eu* 0.14-0.56) indicate crystal fractionation, dominantly of plagioclase and K-feldspar, from a peraluminous magma enriched in F. Isotope data (Sr/Sr = 0.7086, ε = −5.5 to −4.4 with T = 1.5 Ga, zircon ε +0.8 to −6.1; mean T = 1.5 Ga) suggest a Mesoproterozoic continental source, albeit with some younger or more juvenile material indicated by the Hf data. The pluton is the easternmost member of a Carboniferous A-type magmatic suite which shows an increase in juvenile input toward the west in this part of the pre-Andean margin. The petrological and geochemical data strongly suggest a similar intraplate geodynamic setting to that of the nearby but much larger, Late Devonian, Achala batholith, although Hf isotope signatures of zircon suggest a more uniformly crustal origin for the latter. Further studies are required to understand whether these bodies represent two independent magmatic episodes or more continuous activity. [ABSTRACT FROM AUTHOR]

Samar island in the eastern part of Central Philippines is underlain by a complete ophiolite suite, the Samar Ophiolite. We present the first geochronological and paleomagnetic data for the Samar Ophiolite. Whole rock K–Ar dating of two basalt samples yielded an age of 100.2 ± 2.7 Ma and 97.9 ± 2.8 Ma. Thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of D = 340°, I = − 24°, k = 15, α95 = 11° and tilt-corrected direction of D = 342°, I = − 27°, k = 15, α95 = 11°. These values suggest that the ophiolitic basement rocks of Samar formed in the Late Cretaceous at a paleolatitude of 14°S ± 6°. The paleolatitude is several degrees south of the sub-equatorial positions calculated for the three other Mesozoic ophiolites of the Philippine Mobile Belt (PMB) whose paleomagnetism had been previously studied. The PMB ophiolites in eastern and central Philippines share a common age, geochemistry and paleolatitude with the Halmahera Ophiolite, suggesting that they originated from a Mesozoic supra-subduction zone that spanned a few degrees north of the equator to around 15°S. [ABSTRACT FROM AUTHOR]

The Güira de Jauco metamorphic sole, below the Moa-Baracoa ophiolite (eastern Cuba), contains strongly deformed amphibolites formed at peak metamorphic conditions of 650–660°C, approximately 8.6 kbar (~30 km depth). The geochemistry, based on immobile elements of the amphibolites, suggests oceanic lithosphere protholiths with a variable subduction component in a supra-subduction zone environment. The geochemical similarity and tectonic relations among the amphibolites and the basic rocks from the overlying ophiolite suggest a similar origin and protholith. New hornblende40Ar/39Ar cooling ages of 77–81 Ma obtained for the amphibolites agree with this hypothesis, and indicate formation and cooling/exhumation of the sole in Late Cretaceous times. The cooling ages, geochemical evidence for a back-arc setting of formation of the mafic protoliths, and regional geology of the region allow proposal of the inception of a new SW-dipping subduction zone in the back-arc region of the northern Caribbean arc during the Late Cretaceous (ca. 90–85 Ma). Subduction inception was almost synchronous with the main plume pulse of the Caribbean–Colombian Oceanic Plateau (92–88 Ma) and occurred around 15 million years before arc-continent collision (75 Ma–Eocene) at the northern leading edge of the Caribbean plate. This chronological framework suggests a plate reorganization process in the region triggered by the Caribbean–Colombian mantle plume. [ABSTRACT FROM AUTHOR]

The calc-alkaline Bergell intrusion is classically mapped as a main tonalite and main granodiorite unit, whose ages have been determined at 32 and 30 Ma, respectively. These units are separated by a mostly thin band interpreted as magmatic 'transition zone'. High precision U-Pb dating of zircons and whole rock geochemistry of transition zone rocks are combined with a compilation of available geochemical analyses and ages to characterize the emplacement and magmatic evolution of the Bergell intrusion. Detailed field work shows that the transition zone is heterogeneous, characterized by multiple intrusions of tonalites to granodiorites to quartz-monzonites containing mafic enclaves and leucocratic dykes. Commonly, magmatic flow structures such as compositional banding, schlieren and accumulation of K-feldspar megacrystals are observed. The compositions of the transition zone intrusives as well as of the geochemically fairly heterogeneous tonalite and granodiorite units span a continuous range from 49-76 wt% SiO for mafic enclaves to leucocratic dykes. Major and trace element trends are consistent with a predominant fractional crystallization. High precision U-Pb dating of single zircon grains separated from tonalites and granodiorites of the transition zone yields crystallization ages of 31.22 ± 0.04 and 31.13 ± 0.10 Ma, respectively. Hence, the transition zone represents an own phase of intrusion during which a range of magma types were coeval. The available ages and compositional data suggest magmatic activity over the entire range from 28 to 33 Ma across the Bergell intrusion, which should not be described by two main 'tonalite' and 'granodiorite' stages but by a model with several magmatic main phases during which most magma types developed multiply. Geobarometry of tonalite boulders in the Como molasse (yielding a zircon crystallization age of 32.06 ± 0.13 Ma) indicate average erosion rates for the Bergell intrusion of 1.9 mm/a between 32-25 Ma and of 0.5 mm/a since then. [ABSTRACT FROM AUTHOR]

The study of volcanism near unstable plate triple junctions (TJs) offers a unique opportunity to investigate the interactions between mantle dynamics and lithospheric deformation in relation to short-term plate reconfiguration. From combined geochronological and geochemical analyses on Terceira, S. Jorge and Faial volcanic islands, we evidence contrasted modes of melt generation near the Azores Triple Junction during the last 1.3 Myr. The oldest lavas (> 800 ka) erupted along N150 elongated volcanic systems in S. Jorge and Faial have homogeneous isotopic compositions which partly overlap the compositional field of MORBs from the adjacent Mid-Atlantic Ridge (MAR). In contrast, the younger lavas (< 750 ka) erupted along the N110 main structural direction on the three islands are significantly more enriched in LILE and LREE, and have more variable and generally more radiogenic Sr, Pb, Nd and Hf isotopic ratios. Altogether, our data do not support the presence of an active mantle plume under the central Azores. Instead, they suggest that magma generation results from decompression melting of a heterogeneously fertilized mantle (fossil plume?). The higher geochemical heterogeneity of the lavas erupted during the last 750 kyr likely reflects low-degree partial melting promoted by recent reactivation of pre-existing MAR Fracture Zones. We propose that the sub-aerial volcanism over the last 1.3 Myr in the central Azores records a sudden change in the conditions of melt generation, due to a major reconfiguration in regional deformation associated with the recent reorganization of the Eurasia–Nubia plate boundary. [ABSTRACT FROM AUTHOR]

The article discusses the geochronological study of the trachyrhyolites of the Ulkan Group. It states that the volcanic deposits overlie metamorphic rocks of the Early Proterozoic era. It mentions that Riphean deposit sequence is considered as Siberian hypostratotype. It adds that isochronous Rb-Sr method was used to identify the age of the crust located at the base of Lower Riphean Uyan Group. Furthermore, the chemical composition of volcanites correspond to igneous rocks while associated granites and acid volcanites correspond to granites which are A-type.

Bowers Ridge is an ~700 km long arcuate ridge behind the Central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian—Bering Sea region. Here we present the first geochemical and 40Ar/40Ar age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V Sonne cruise SO201-1b. The northern Bowers Ridge basement (26-32 Ma) consists of mafic to intermediate calc-alkaline rocks with adakite-like (Sr/Y = 33-53, LaN/YbN = 3.3-7.8), high field strength element (HFSE)—depleted (e.g., NbN/LaN = 0.07-0.31) trace element patterns and Sr-Nd-Pb isotope compositions within the Western Aleutian Arc array, implying magma generation above an obliquely subducting slab. The seamount samples (22-24 Ma) are HFSE-rich alkaline olivine basalts (LaN/YbN = 3.3-3.9, NbN/LaN = 1.0-1.4) with minor arc-type trace element signatures (PbN/CeN = 1.4-1.6, KN/NbN = 1.7-1.9) but with Pacific mid-oceanic-ridge basalt (MORB)—like isotopic compositions, pointing to an origin by small-degree decompression melting from slightly subduction-modified mantle. The geochemistry of the recovered rocks can be explained by highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as a Cenozoic island arc. [ABSTRACT FROM AUTHOR]

Abstract: The Tianyu mafic–ultramafic intrusion is one of many important sulfide-bearing mafic–ultramafic intrusions in the eastern Tianshan terrane located in the southern margin of the Central Asian Orogenic Belt in north Xinjiang, NW China. The origin of sulfide mineralization in these intrusions, their relationship with Permian basalts and A-type granites, and the geodynamic setting of the bimodal magmatism in the region are controversial. In this paper we use zircon U–Pb age, Hf–Sr–Nd–Os isotope and lithophile–chalcophile trace element constraints from the Tianyu mafic–ultramafic intrusion to address these important issues. The U–Pb age of zircon from the Tianyu intrusion determined by SIMS is 280±2Ma. This confirms that the Tianyu intrusion is coeval with most sulfide-bearing mafic–ultramafic intrusions and some basalts and A-type granites in the Tianshan region. Similar to coeval basalts in the nearby Tuha basin, the Tianyu intrusive rocks are characterized by negative Nb anomaly and elevated εNd values varying between −1 and +4.6. The negative Nb anomaly in the Tianyu intrusion is not as pronounced as in the coeval A-type granites but the εNd values for these bimodal rocks are similar. The εHf values of zircon from the Tianyu intrusion are positive, varying between 3 and 8. These values suggest that the parental magma of the Tianyu intrusion was dominated by melt derived from a depleted source mantle. The γOs values of pyrrhotite from the Tianyu intrusion are high, ranging from 725 to 995, which can be modeled by selective assimilation of sulfide from the lower and upper crusts. Negative Pt anomaly is present in relatively sulfide-rich samples such as semi-massive and massive sulfide ores but not in sulfide-poor samples, which is more consistent with the effect of post-magmatic hydrothermal alteration than the original signature of the parental magma. The results of modeling using the compositions of selected disseminated sulfide samples suggest that the parental magma of the Tianyu intrusion was depleted in all PGE, which is consistent with previous sulfide segregation at depth. Our data and modeling results support the model of decompression melting of upwelling asthenosphere due to slab break-off during a transition from oceanic subduction to arc–arc or arc–continent collision in the Permian. Decompression melting of the upwelling asthenosphere in the region produced mafic magma. Partial melting in the lower parts of a juvenile arc crust due to mafic magma underplating produced granitic melts. Mixing of the mafic magma with a small amount of granitic melts at depth followed by contamination with the upper crust at a higher level formed the parental magma of the Tianyu intrusion. PGE depletion in the parental magma of the Tianyu intrusion can be attributed to sulfide segregation associated to the first stage of crustal contamination at depth. We suggest that the formation of sulfide ores in the intrusion was related to the second stage of crustal contamination in the upper crust. [Copyright &y& Elsevier]

Two age groups were determined for the Cenozoic granitoids in the Dinarides of southern Serbia by high-precision single grain U-Pb dating of thermally annealed and chemically abraded zircons: (1) Oligocene ages (Kopaonik, Drenje, Željin) ranging from 31.7 to 30.6 Ma (2) Miocene ages (Golija and Polumir) at 20.58-20.17 and 18.06-17.74 Ma, respectively. Apatite fission-track central ages, modelling combined with zircon central ages and additionally, local structural observations constrain the subsequent exhumation history of the magmatic rocks. They indicate rapid cooling from above 300°C to ca. 80°C between 16 and 10 Ma for both age groups, induced by extensional exhumation of the plutons located in the footwall of core complexes. Hence, Miocene magmatism and core-complex formation not only affected the Pannonian basin but also a part of the mountainous areas of the internal Dinarides. Based on an extensive set of existing age data combined with our own analyses, we propose a geodynamical model for the Balkan Peninsula: The Late Eocene to Oligocene magmatism, which affects the Adria-derived lower plate units of the internal Dinarides, was caused by delamination of the Adriatic mantle from the overlying crust, associated with post-collisional convergence that propagated outward into the external Dinarides. Miocene magmatism, on the other hand, is associated with core-complex formation along the southern margin of the Pannonian basin, probably associated with the W-directed subduction of the European lithosphere beneath the Carpathians and interfering with ongoing Dinaridic-Hellenic back-arc extension. [ABSTRACT FROM AUTHOR]

Based on studies of the geochemical elements of core sediments collected from the subaqueous Yangtze River delta front, combined with 210Pb and 137Cs nuclide dating, element recording profiles were set up and anthropogenic impacts on the sedimentary records were considered. The results suggest that (1) The core formed since 1945, of homogeneous clay and silt sediments. (2) The content of most elements underwent little change, but the content of S and Nb increased gradually. In particular concentrations of these elements and heavy metals increased rapidly and were maintained at high levels in the surface and sub-surface layers (i.e. since the 1990s). The elements S, As and Nb and principal factor 4 (F4), which were sensitive to anthropogenic impacts, could be used as indicators of human activities. (3) The value of each indicator has increased gradually since 1945, with a sharp increase over the last 20 years. When divided according to changing values of the indicators at 185 cm (1955), 97 cm (1978), 47 cm (1992), and 7 cm (2003), five periods of the geochemical elements records were obtained. These corresponded to the ages of from the Chinese Civil War to the early stages of planned economy, the implementation of planned economy, the reform and opening-up period, the market economy establishment phase and the environment control and protection stage after 2003, respectively. [ABSTRACT FROM AUTHOR]

The Huashanguan rapakivi pluton in Zhongxiang, Hubei Province, China, is the first discovered Proterozoic rapakivi pluton in the Yangtze block. Based on field and petrographical observations, a typical rapakivi texture was found in the northern portion of the Huashanguan granitic pluton. Almost all the K-feldspar phenocrysts were round to oval in shape and most had plagioclase coatings known as rapakivi phenocrysts. Alkali feldspars and quartz had two or more generations. Petrochemically, the Huashanguan rapakivi granites were characterized as having high values of Si, K, Fe, Th, U, La, Ga, Ce, Sm and LREE, low values of Ca, Mg, Sr, Nb, Y and HREE, and a negative Eu anomaly. These geochemical characteristics of the Huashanguan granites were concordant with typical rapakivi granites, and had an affinity to A-type granites. LA-ICP-MS U-Pb zircon dating also was conducted. The dating yielded a207Pb/206Pb weighted mean age of 1851±18 Ma (MSWD =1.2), which represents the age of the pluton emplacement. The age of 803±170 Ma at the lower intercept in the concordia diagram corresponds to the age of a later deformation event which affected the pluton, and suggests that the Huashanguan pluton was influenced by Neoproterozoic thermo-tectonic events after its formation. The discovery of Paleoproterozoic Huashanguan rapakivi granites indicates continental rifting or a post-orogenic extensional event that took place in the Paleoproterozoic in the Yangtze block. These events may be related to the breakup of the Paleoproterozoic Columbia supercontinent. [ABSTRACT FROM AUTHOR]

The article presents a study on the age determination of the rare earth element (REE) distribution in Zircon from a granitoid sample from the Southern Prince Charles Mountains in Eastern Antarctica. It states that the presence of Ruker granite in the area indicates a series of felsic orthogneiss, metasedimentary and volcanic events. The researchers used a titanium geothermometer to estimate the crystallization temperature of zircon in the area. It concludes that the Sodruzhestva Series in the area was formed in the post-Archean time because signs of Archean - Paleoproterozoic tectonothermal processes are absent in the samples.

The article discusses the method of isotope-geochronological timing of rock metamorphism at the boundary zone of Aldan Shield and the Dzhugdzhuro-Stanovoi Folded Area in the area of Nagornyi Village and Kholodnikan Pass in Russia. The method indicated an overthrust-sheet structure that developed in the zone and a tectonic plate that moderately points to the south. Further observations showed that the metamorphic rocks are deformed linearly or folded isoclinally in the latitudal direction parallel to the whole junction zone.

The article discusses the study on the mineralogy, geochemistry and age peculiarities of zircon distribution on gneissose leucocratic miaskites of the Ilmenogorsk massif deposit at the southern region of Ural Mountains in Russia. Researchers conducted recrystallization of the zircon samples from amphibole miaskites that yielded formation of emulsion structures, crushing and corrosion, and new growth. From the authors' findings, ages of miaskite crystallization, 434 plus 15 minus 10 million years (Ma), and metamorphism, 239 plus 40 minus 45 Ma, were determined by standard uranium-lead (U-Pb) dating method.

Abstract: The chronology and origin of volcanism of Ascension Island, South Atlantic Ocean, is poorly resolved. Here we use in situ produced cosmogenic 3He in olivine and pyroxene phenocrysts from well-preserved lava flows to date the main sub-aerial basalt volcanism on the island. Etching olivine separates in HF/HNO3 appears to remove a significant proportion of the implanted radiogenic 4He contribution. Average exposure ages of each flow corrected for radiogenic He range from 328ka to 186ka and are used to refine the chronology and stratigraphy of the island. Magmatic 3He/4He ratios derived from in vacuo crushing are in the range of 6.3–7.3 R A. This range is lower than the neighbouring Mid-Atlantic Ridge segment (6–8°S) but slightly higher than measured in regional ocean islands of St. Helena, Tristan da Cunha and Gough. Combining these data with new trace element data and published radiogenic isotope ratios it appears that the Ascension Island magmatism is a mix of HIMU mantle material, typified by basalts from St. Helena, and depleted MORB-source mantle. [Copyright &y& Elsevier]

Abstract: This study focuses on denitrification in a sandy aquifer using geochemical analyses of both sediment and groundwater, combined with groundwater age dating (3H/3He). The study sites are located underneath cultivated fields and an adjacent forested area at Oostrum, The Netherlands. Shallow groundwater in the region has high nitrate concentrations (up to 8mM) due to intense fertilizer application. Nitrate removal from the groundwater below cultivated fields correlates with sulfate production, and the release of dissolved Fe2+ and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the nitrate removal zone, indicate that denitrification coupled to pyrite oxidation is a major process in the aquifer. Significant nitrate loss coupled to sulfate production is further confirmed by comparing historical estimates of regional sulfate and nitrate loadings to age-dated groundwater sulfate and nitrate concentrations, for the period 1950–2000. However, the observed increases in sulfate concentration are about 50% lower than would be expected from complete oxidation of pyrite to sulfate, possibly due to the accumulation of intermediate oxidation state sulfur compounds, such as elemental sulfur. Pollutant concentrations (NO3, Cl, As, Co and Ni) measured in the groundwater beneath the agricultural areas in 1996 and 2006 show systematic decreases most likely due to declining fertilizer use. [Copyright &y& Elsevier]

The Zona Canteras-Puerto Peñasco area, previously known as Seven Hills, is located ~20 km ENE of the town of Puerto Peñasco in NW Sonora, Mexico. Crystalline basement in this area is key to defining Proterozoic provinces of SW Laurentia and has important implications for Phanerozoic tectonics and formation of the active continental margin of North America. New U-Pb zircon ages (LA-MC-ICPMS) of two banded gneisses in this area yield 207Pb/206Pb ages at 1764 ± 3 Ma and 1725 ± 3 Ma. Zircons from both samples are well-behaved in the U-Pb systematics and present age and compositional homogeneity, suggesting that these rocks are Paleoproterozoic orthogneisses. These crystallization ages combined with whole rock major- and trace-element geochemistry and Sm-Nd in whole rock and Lu-Hf isotopic determinations on zircons by laser ablation suggest that the banded gneisses have affinities with rocks of the Yavapai province as defined in the Quitovac area ~45 km to the east and in the region of Gunnison-Salida, Colorado, USA. Both samples plot in the volcanic arc environment of granite formation (Y vs Nb discrimination diagram) and have epsilon Hf (initial) values between +1.85 and +5.05 with corresponding single-stage Hf model ages (TDM) at 1.98-2.07 Ga and two-stage Hf model ages (TDMC) at 2.07-2.50 Ga. Sm-Nd whole rock isotopic analyses from three Paleoproterozoic gneisses have epsilon Nd (initial) values between +1.86 and -0.57 with corresponding Nd model ages (TDM) at 1.91-2.02 Ga. The magmatic protoliths of these gneisses could have been formed mostly from mantle sources with some degree of crustal contribution, which is similar to rocks from the Yavapai province in SW North America. This petrogenesis contrasts with typical Mojave-province rocks that are characterized by significantly greater crustal contamination (as much as 30-40% in Death Valley). If these Mojave province rocks exist in NW Sonora they must be located to the west of the Zona Canteras-Puerto Peñasco area. [ABSTRACT FROM AUTHOR]

Abstract: Instrumental and spectral characteristics germane to chemical dating of monazite have been tested using the Cameca SX-100 at Rensselaer Polytechnic Institute. Statistical analysis demonstrates that, for trace element analysis, equal counting time on peak and background is required for optimal statistical precision, thus rendering impractical the procedure of fitting the entire spectrum to obtain background values. Energy shifts require shifting the detector voltage window between peak and background positions, and it is concluded that the differential auto PHA mode works optimally for this. Analyses of Pb-free phosphates, silicates, and oxides are used to measure spectral interferences with the PbMα peak and background positions. Backgrounds were modeled using both linear and exponential fits. It was found that the difference in background counts using the two fits varies with each of the five spectrometers examined, and that the high-pressure (3bar) detectors show larger differences in exponential vs. linear peak-minus-background (P-B) values than the low-pressure (1bar) detectors. In addition, every spectrometer requires a unique correction for every major element in monazite. An analytical protocol is presented that incorporates these results. This protocol was applied to several monazite standards to determine inter-spectrometer variability, and spectrometer reproducibility from session to session. It was found that the difference in composition (and age) between spectrometers on identical spots exceeds the 2 sigma standard error of the mean of composition (or age) on either spectrometer. This means that (a) additional sources of error beyond the counting statistics exist between spectrometers; (b) the precision of microprobe ages cannot be continuously improved by additional counting; and (c) the minimum realistic precision is on the order of ±2–3% for monazites with around 1500–2000ppm total Pb, or an additional absolute uncertainty of 20–50ppm Pb. [Copyright &y& Elsevier]

Abstract: Detection and precise quantification of the different carbonate minerals is crucial in selecting coral reef samples prior to geochemical and radiochronological measurements, thus helping to interpret changes in the mineralogical composition of geological formations made of reef material and associated sediments such as periplatform oozes. While powder X-ray diffraction (XRD) is a widely used method for mineralogical analysis, precise and accurate quantification of mineral abundance requires a thorough calibration of the instrument and method by means of gravimetric standards. In this study, we optimize a calibration method for the quantitative determination of four widespread Ca and Mg carbonates: calcite, aragonite, magnesian calcite (Mg-calcite) and dolomite. To detect and quantify very low calcite contents, which provide a crucial screening criterion for coral radiochronology, we perform a detailed survey of analytical precision and limits using two different XRD facilities. Detection and quantification limits (≈0.18% and ≈0.9% calcite in a calcite–aragonite mixture, respectively) are shown to be similar between the two instruments. The reproducibility of measurements clearly demonstrates that it is preferable to use peak area rather than peak height alone to obtain a precise quantification of the abundances of various carbonate minerals. [Copyright &y& Elsevier]