Search

The geochronological, geochemical, and Sr–Nd isotopic features of the Late Mesozoic volcanic rocks associated with the development of the Nercha Depression in the Eastern Transbaikalia are presented. The structure of the depression involves the sequences of K-rich subalkaline volcanic rocks with content of 55 to 73 wt % SiO2.40Ar/39Ar isotope dating of bulk rock samples was conducted. 40Ar/39Ar dating gives a value of 150.8 ± 1.8 Ma for K-rich andesibasalt and 131.0 ± 1.6 Ma for latite. Volcanic rocks are characterized by depletion of hydrophobic Ti, Nb, and Ta, slightly negative εNd(T) values, but increased in εSr(T) values, which probably indicates contamination processes of primary melts by the crust component. In terms of their geological and geochemical characteristics, the volcanic rocks of the Nercha Depression belong to the shoshonite–latite series of the rear part of the Greater Khingan volcanic region, which formed in the subduction setting of the continental active margin. [ABSTRACT FROM AUTHOR]

High-pressure and ultra-high-pressure metamorphic terrains display an internal architecture consisting of a pile (or stack) of several coherent tectonic thrust sheets or units. Their identification is fundamental for understanding the scale and mechanisms active during subduction and exhumation of these crustal slices. This study investigates the geometry of the northern Dora-Maira Massif and the kinematics of the major tectonic boundaries, combining field and geochronological data. The tectonic stack of the northern Dora-Maira Massif comprises the following units. The lowermost unit (the Pinerolo Unit) is mainly characterized by Upper Carboniferous fluvio-lacustrine (meta-)sediments. The Pinerolo unit is overthrust by a pre-Carboniferous basement. The latter is subdivided in two tectonic units (the Chasteiran and Muret Units) with different Alpine metamorphism (ultra-high-pressure and high-pressure, respectively). The pre-Carboniferous basement of the Muret Unit is thicker than previously thought for two main reasons. Firstly, some paragneisses, traditionally assumed to be Carboniferous and/or Permian in age, display detrital zircon ages indicating a main source at about 600 Ma. Secondly, three samples of the Granero Orthogneiss, previously assumed to be a Permian intrusive body, have provided zircon U–Pb ages of 447 ± 1 Ma, 456 ± 2 Ma and 440 ± 2 Ma, indicating a late Ordovician or early Silurian age for the protoliths. The uppermost unit (the Serre Unit) comprises porphyritic (meta-) volcanic and volcaniclastic rocks dated to the Permian (271 ± 2 Ma), on top of which remnants of the Mesozoic cover is preserved. Detailed mapping of an area about 140 km2 shows that (i) the ultra-high pressure Chasteiran Unit is localized at the boundary between the Pinerolo and Muret Units, (ii) the Granero Orthogneiss may be considered as the mylonitic sole of the Muret Unit, characterized by a top-to-W sense of shear, and (iii) the contact between the Muret and Serre Units displays ductile-to brittle structures (La Fracho Shear Zone), indicating a top-to-the-NW displacement of the hangingwall with respect to the footwall. A final episode of brittle faulting, cutting across the nappe stack (the Trossieri Fault), indicates an extensional stage in the core of the Alpine belt, as previously documented in more external zones. This work provides a necessary and robust basis before an accurate discussion of processes acting during continental subduction of the Dora-Maira Massif may be understood. [ABSTRACT FROM AUTHOR]

In this study, zircon U‐Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone (YZSZ) in southern Xizang (Tibet) yielded an age of 247 ± 3 Ma. According to whole rock geochemical and Sr‐Nd‐Pb isotopic data, the Early Triassic samples could be divided into two groups: Group 1 with P‐MORB affinity, showing initial 87Sr/86Sr ratios of 0.70253–0.70602, εNd(t) values of 4.2–5.3, (206Pb/204Pb)t ratios of 16.353–18.222, (207Pb/204Pb)t ratios of 15.454–15.564, and (208Pb/204Pb)t ratios of 35.665–38.136; Group 2 with OIB affinity, showing initial 87Sr/86Sr ratios of 0.70249–0.70513, εNd(t) values of 4.4–4.9, (206Pb/204Pb)t ratios of 17.140–18.328, (207Pb/204Pb)t ratios of 15.491–15.575, and (208Pb/204Pb)t ratios of 36.051–38.247. Group 2 rocks formed by partial melting of the mantle source enriched by a former plume, and assimilated continental crustal material during melt ascension. The formation of Group 1 rocks corresponds to the mixing of OIB melts, with the same components as Group 2 and N‐MORBs. The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo‐Tethys in southern Xizang (Tibet). [ABSTRACT FROM AUTHOR]

High-resolution dating of volcanic rocks is the foundation for understanding the evolution of volcanoes and for estimating possible hazards. However, dating is often difficult for submarine volcanoes, where radiocarbon or other dating is frequently unavailable or imprecise. Here, we report paleointensity results from submarine basalts around Izu-Oshima Island, a typical island-arc volcano, and their bearing on age constraints. Basaltic lava samples were collected from submarine ridges located southeast of Izu-Oshima Island. Rock magnetic data indicate that the samples contain Ti-rich titanomagnetite with blocking temperatures of around 250–400 °C. The magnetic properties of the samples do not change significantly when heated in Ar or vacuum. We apply the Tsunakawa-Shaw method to estimate absolute paleointensity. Samples from different submarine ridges show distinct behavior. One ridge (SE1) shows moderate paleointensity of about 37 μT, while another ridge (SE3) records relatively strong magnetic fields of about 60 μT. Comparing those results with regional paleointensity data, we estimate the age of the SE1 ridge to be younger than 0.5 ka or around 1.4 ka. The other ridge (SE2) exhibits different paleointensity for two samples obtained from different localities, implying that the ridge consists of multiple eruptions. These results demonstrate that paleomagnetism can improve the dating of submarine volcanic rocks. [ABSTRACT FROM AUTHOR]

There has been no consensus yet regarding the precise initial timing and duration of the late Mesozoic extensional tectonics in the South‐eastern China Block. This work focusses on the growth strata of the Early Cretaceous red beds in the South‐eastern China Block to determine the late Mesozoic tectonics and the precise timing of the initiation and duration of extensional tectonics in this area. Field observation of several terrigenous basins shows that the dip angles of the Cretaceous red beds have varied from moderate to gentle from basin edges to interiors (or centres). The visible and estimated thickness within a single bed increases slightly downwards from the upper to the lower part. These characteristics indicate that the sedimentary area of these beds has undergone an extensional process with expansion and deepening of the sedimentary basins. Rotation of the border surfaces (limbs) and downward warping of the hanging walls or retreat of the footwalls of listric normal faults causes three types of extensional growth (or syntectonic) strata in the deposits of different basins. Dating of the volcanic rocks related to the growth beds reveals that the sedimentary basins were enlarged and deepened when the Early Cretaceous strata were deposited in the South‐eastern China Block from ca 140 to 137 Ma. Regionally, under the influence of Palaeo‐Pacific plate rollback since ca 140 Ma, the South‐eastern China Block stress field has led to lithospheric uplift and pull‐apart structures near the surface, causing the half‐graben basins to receive sedimentation. Although the extensional event was interrupted by a short compressional event during 120 to 105 Ma, with the oceanward retreat of the trench, the area of extension gradually enlarged and rejuvenated south‐eastwards until the end of the Cretaceous. This Cretaceous extension event of the South‐eastern China Block must belong to a worldwide geological event with global significance. [ABSTRACT FROM AUTHOR]

The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west. A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardlydeformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples. Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study. The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province. [ABSTRACT FROM AUTHOR]

The drift history of the Lhasa terrane from Gondwana to Asia plays a crucial role in understanding the Tethys evolution and true polar wander (TPW). However, few reliable paleomagnetic results from Jurassic strata are currently available for reconstructing its northward journey. We performed a combined paleomagnetic and geochronological study on Bima Formation strata in the Xigaze area. Combined with previous results from the Sangri area, our results reveal a paleolatitude of 8 ± 4°S at ∼180 Ma for the reference point (29.3°N, 90.3°E). Along with other paleomagnetic results from the Triassic to Cretaceous, our new results suggest that the Lhasa terrane motion accelerated from ∼2 cm/yr during ∼220–180 Ma to ∼17 cm/yr during ∼180–170 Ma. Paleolatitude information of the North Qiangtang terrane and Tethyan Himalaya is calculated from paleopoles that meet five criteria, which include (a) structural control, (b) well‐determined rock age, (c) stepwise demagnetizations, (d) a minimum of 25 specimens or 8 sites are contained, and (e) robust field or reversal tests are provided. Both terranes also show significant acceleration during their northward motion, which may be related to oceanic slab subduction. Thus, all Gondwana‐derived microcontinents seem to share a significant acceleration during their northward motion. In addition, recent paleomagnetic results from volcanic rocks dated at ∼155 Ma subdivide the overall northward motion during ∼170–130 Ma into two stages, which include a southward drift during ∼170–155 Ma followed by northward motion during ∼155–130 Ma. These results support the fast Late Jurassic TPW during a ∼10 Myr time span. Plain Language Summary: The Tibetan Plateau is composed of the Qiangtang, Lhasa, and Himalaya terranes that originate from Gondwana, which was an ancient supercontinent located mainly in the southern hemisphere. How these terranes moved northward from Gondwana to Asia remains unclear due to poor paleolatitude constraints. We provide paleomagnetic evidence to show that the Lhasa terrane was located at ∼8°S at ∼180 million years ago (Ma). Together with reliable paleomagnetic data from Late Triassic (∼237–201 Ma) limestones and Middle Jurassic (∼174–163 Ma) sandstones, our results suggest a significant acceleration during Lhasa's northward motion at ∼220–170 Ma. Reliable paleolatitude data show significant accelerations of the Lhasa, North Qiangtang, and Tethyan Himalaya terranes during their northward journey, which may be related to oceanic slab subduction. These significant accelerations may be a common feature of these Gondwana‐derived microcontinents. The drift history of the Lhasa terrane during ∼170–130 Ma can be subdivided into a southward drift during ∼170–155 Ma and a subsequent northward drift during ∼155–130 Ma. The yoyo‐like drift motion of the Lhasa terrane near the equator before and after ∼155 Ma supports a fast Late Jurassic pole motion event (named true polar wander) during a ∼10 Myr time span. Key Points: The Lhasa terrane was located at ∼8°S at ∼180 Ma for the reference point (29.3°N, 90.3°E)Lhasa terrane's motion accelerated from ∼2 cm/yr during ∼220–180 Ma to ∼17 cm/yr during ∼180–170 MaThe Lhasa terrane underwent a yoyo‐like drift motion near the equator at ∼170–130 Ma, supporting the fast Late Jurassic true polar wander [ABSTRACT FROM AUTHOR]

The Xiong'er Group is mainly distributed in western Henan, southern Shanxi, and southeastern Shaanxi. As a set of low-grade metamorphic and low-deformation volcanic rocks formed in the transition period during the tectonic evolution of the North China Craton in the Precambrian, the tectonic and magmatic evolution information of this period are well recorded. The accurate isotopic dating of the Xiong'er Group has great significance to the study of the stratigraphic division and formation of volcanic rocks in the Xiong'er Group and the improved cognition of regional tectonic magmatism. In this study, through the SHRIMP (sensitive high-resolution ion microprobe) zircon uranium-lead (U-Pb) isotope dating of the volcanic rocks of the Xiong'er Group in the Xiong'er Mountain, the research results showed that the age data were separated into two intervals, the magmatic zircon ages from 1836 to 1711 Ma and the inherited zircon ages from 2415 ± 19 to 2193 ± 34 Ma. The isotopic age of magmatic zircon was considered to represent the formation age of the Jidanping Formation in the Xiong'er Mountain, and the isotopic ages of the inherited zircons were consistent with the formation ages of the Taihua Group supracrustal rocks in the lower Xiong'er Group. Combined with previous studies on isotope and rock geochemistry of volcanic rocks in the Xiong'er Group, it was believed that the formation ages of the volcanic rocks of the Xiong'er Group are between 1874 and 1618 Ma, among the ages of the Xushan Formation, Jidanping Formation, and Majiahe Formation, which were concentrated between 1874 and 1800 Ma, 1836 and 1711 Ma, and 1780 and 1618 Ma, respectively. The age of inherited zircon in the volcanic rocks of the Xiong'er Group indicated that the formation of the volcanic rocks of the Xiong'er Group had the addition of stratigraphic materials from the Taihua Group. [ABSTRACT FROM AUTHOR]

High-grade epithermal Au–Ag veins of the Vodorazdelnaya district in far eastern Russia are hosted by Early Cretaceous volcanic rocks of the Tytylveem belt. The largest deposit is Zone 37, a 1-km long, northeast-striking vein that is up to 35-m wide, containing at least 32 t Au. The main shoot consists of crustiform—colloform banded quartz-chalcedony-adularia-sulfide veins, of low-sulfidation style. The vein is cut by late-mineral rhyolite sills and dikes. Zone 37 vein is dated at 117 ± 2 Ma, overlapping within error with the cross-cutting rhyolites (121–119 Ma) and with intrusions of the nearby Ilirney granitic pluton (119–117 Ma). The volcanic host rocks are dated at 121–115 Ma. At September Northeast, 15 km west-northwest of Zone 37, mineralisation consists of veins up to 4 m wide, with crustiform banded quartz ± chalcedony ± adularia ± chlorite, and Zn-Pb-Cu sulfides, of intermediate sulfidation style, in a 1-km-long, north-northeast striking trend. High grades are associated with gold dendrites in fine-grained quartz bands and with minor late-stage tellurides. The veins were disrupted by intrusion of rhyolite dikes and associated phreatic breccias. Some breccias host mineralisation as vein clasts and minor matrix sulfides. The Tytylveem belt is preserved below a regional unconformity that forms the base to the overlying Late Cretaceous (106–77 Ma) Okhotsk-Chukotka Volcanic Belt (OCVB). Intrusive rocks attributed to OCVB magmatism were emplaced at about 96 Ma and are associated with overprinting hydrothermal alteration, sub-economic veins and thermal resetting of some vein adularia ages at Zone 37, to 91–97 Ma. [ABSTRACT FROM AUTHOR]

New U–Pb and 40Ar/39Ar isotopic dates on volcanic rocks of the East Chukotka segment of the Okhotsk–Chukotka Belt correspond to 76–71 Ma span, which is partially beyond the conventional lifetime of this volcanic province. This could result from the migration of magmatic loci along the strike of the volcanic belt, and/or from the overprint by a younger magmatic event. [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]

The West Pekulnei marginal continental arc terrane adjoins the East Pekulnei oceanic crust terrane, the siliceous–volcanic strata of which (Pekulneiveem Formation, PV) are subdivided into two complexes. The first PV-1 complex comprises a dike series; pillow basalts with elevated Nb and TiO2 contents and FeO*/MgO ratios; and siliceous–argillaceous deposits dated as Bajocian–Kimmeridgian. The second PV-2 complex consists of low-Ti, N-MORB-type pillow basalts with a low Nb content, combined with magnesian basalts, picrite–basalts, and mafic tuffaceous rocks. The siliceous–argillaceous deposits of the PV-2 complex are dated as Late Jurassic–Early Cretaceous based on radiolarian assemblages and contain Valanginian macrofauna; the admixture of terrigenous material and the detritus of intermediate and felsic volcanic rocks suggest the PV-2 complex generation in the vicinity of a source of allothigenic material. New data on the geology and geochemical composition of the pillow basalt sequence suggest the formation of the PV-1 complex in a Paleo-Pacific spreading center. The composition and datings of the rocks of the PV-2 complex indicate its formation during the Tithonian–Valanginian in the junction zone of the oceanic plate and the Pekulnei–Zolotogorsk margin of the Chukotka continent. The formation of the PV-2 complex immediately after the convergence of the Paleo-Pacific plate (PV-1) with the continental margin is explained by the young PV-2 oceanic crust generation in a spreading center, which emerged in a transform fault zone. The Pekulnei arc formed during the Tithonian–Valanginian at the Pacific margin of Eastern Chukotka; the magmatic complexes of this arc, besides the IAT and CA series, include low-Ti MORB-type basalts, similar in composition to a certain part of the PV-2 basalts. Presumably, a single tectonic structure existed in the Tithonian–Valanginian that controlled the basaltic magmatism of both terranes; this structure could be a transtensional segment of the transform fault zone. The synchronous development of the Pekulnei arc and PV-2 complexes and the geochemical correlation between the basalts suggest that the Tithonian–Valanginian subduction within the Pekulnei segment was induced by the opening of the PV-2 basin in the transform fault zone and the subduction of the young oceanic crust beneath the margin of Eastern Chukotka. [ABSTRACT FROM AUTHOR]

The Lengshuikeng Ag‐Pb‐Zn ore field is located in the North Wuyi Mesozoic volcanic belt south of the Qinzhou–Hangzhou suture zone between the Yangtze and Cathaysia paleo‐plates. Previous zircon U–Pb geochronological studies on ignimbrites and tuffs from this area have yielded conflicting ages of 157–161 Ma (Early Upper Jurassic) and 137–144 Ma (Early Lower Cretaceous). Volcanic rocks in the ore field have even been proposed to include both ages. Our SHRIMP zircon U–Pb dating of the ignimbrite and tuff samples from the ore field, along with field observations and results from geochronological work on other volcanic and sub‐volcanic rocks in the region, shows that two populations of magmatic zircons, one autocrystic and the other xenocrystic, are present in the pyroclastic rocks. The autocrystic zircons have ages suggesting formation/eruption at approximately 140 Ma, whereas the xenocrystic zircons give ages of 155–159 Ma, indicating intrusion of granitic porphyries in the Early Upper Jurassic. Therefore, the pyroclastic rocks in the Lengshuikeng Ag–Pb–Zn ore field formed in the Early Lower Cretaceous. The youngest zircon U–Pb ages from pyroclastic rocks may not represent the formation/eruption ages of the host rock, depending most likely on the existence and/or abundance of juvenile or vitric pyroclasts in the rocks. [ABSTRACT FROM AUTHOR]

A co-investigation of mantle melting conditions and seismic structure revealed an evolutionary record of mantle dynamics accompanying the transition from subduction to collision along the Africa-Eurasia margin and the >1 km uplift of the Anatolian Plateau. New 40Ar/39Ar dates of volcanic rocks from the Eastern Taurides (southeast Turkey) considerably expand the known spatial extent of Miocene-aged mafic volcanism following a magmatic lull over much of Anatolia that ended at ca. 20 Ma. Mantle equilibration depths for these chemically diverse basalts are interpreted to indicate that early to middle Miocene lithospheric thickness in the region varied from ∼50 km or less near the Bitlis suture zone to ∼80 km near the Inner Tauride suture zone. This southward-tapering lithospheric base could be a vestige of the former interface between the subducted (and now detached) portion of the Arabian plate and the overriding Eurasian plate, and/or a reflection of mantle weakening associated with greater mantle hydration trenchward prior to collision. Asthenospheric upwelling driven by slab tearing and foundering along this former interface, possibly accompanied by convective removal of the lithosphere, could have led to renewed volcanic activity after 20 Ma. Melt equilibration depths for late Miocene and Pliocene basalts together with seismic imaging of the present lithosphere indicate that relatively invariant lithospheric thicknesses of 60–70 km have persisted since the middle Miocene. Thus, no evidence is found for large-scale (tens of kilometers) Miocene delamination of the lower lithosphere from the overriding plate, which has been proposed elsewhere to account for late Miocene and younger uplift of Anatolia. [ABSTRACT FROM AUTHOR]

In this work, we report the geochemical, whole-rock Sr-, Nd-, and Hf-isotopes, and zircon U–Pb age and Hf–O isotopic compositions for six granitic intrusions in northeastern Fujian, coastal region of southeastern China, to elucidate the petrogenesis of aluminous and peralkaline A-type granites in this region. Zircon U–Pb dating of these rocks (including porphyritic syenites, porphyritic granites, alkali-feldspar granites, and arfvedsonite granites) yielded Late Cretaceous ages of 94–98 Ma. Mineral assemblage and geochemical features suggested that the arfvedsonite granites were peralkaline A-type but the porphyritic syenites, porphyritic granites, and alkali-feldspar granites were aluminous A-type. Geochemical data, whole-rock Nd–Hf isotopes (ε Nd (t) = −5.5 to −4.0, ε Hf (t) = −3.6 to 0), and the zircon Hf–O isotopes (ε Hf (t) = −3.0 to +1.4, δ18O = 5.4‰–6.0‰) indicated that the porphyritic syenites were produced by partial melting of crustal rocks in the lower crust. The porphyritic granites and alkali-feldspar granites had similar geochemical features and zircon Hf–O isotopic composition with the porphyritic syenites, suggesting that they were formed by feldspar-dominant crystal fractionation of deep crust derived syenitic magma under shallow crustal level. Compared with the aluminous A-type granites, the peralkaline A-type granites had lower abundance of Al 2 O 3 , MgO, CaO, Ba, and Sr, higher abundance of SiO 2 and HFSE, as well as higher ratio of total FeO/MgO, Ga/Al, and Rb/Sr. However, the whole-rock Nd-, Hf- and zircon Hf–O isotopic compositions of the peralkaline and aluminous A-type granites were similar. It could be inferred that the peralkaline A-type granites were formed by crystal fractionation of aluminous A-type granitic magmas. Since the studied A-type granitic intrusions were coeval with the bimodal volcanic rocks and the extensional basins in the coastal region of southeastern China, they were likely produced in an extensional setting, possibly during lithospheric thinning that resulted from the subduction of the paleo-Pacific plate beneath southeastern China. • Aluminous and peralkaline A-type granites were crystallized at 94–98 Ma. • Aluminous and peralkaline A-type granites were derived from a common source. • Aluminous A-type granite was produced by partial melting of lower crustal rocks. • Peralkaline A-type granite was formed via crystal fractionation. [ABSTRACT FROM AUTHOR]

In this paper, we describe gravity flow facies that form the base of the Meiji Reef system in the South China Sea, and discuss their palaeoclimate significance. These Early Oligocene (26.1–23.5 Myr) deposits occur in Well NK1 and comprise a 110.6 m thick succession developed on top of volcanic basement. The deposits can be divided into two intervals, both of which are dominated by debris flow facies. The lower 17.6 m thick interval has a gravel content of 50–65% with parent rocks including dacite, basalt, tuff, and coral reef fragments. From base to top, these units coarsen upward and show increasing gravel contents. Gravel clasts are well-rounded but poorly sorted (diameter 10–350 mm), which, together with their complex composition, indicate transportation by the gravity flows from the near-shore volcanic rock basement to the slope. The upper 93 m thick interval comprises abundant bioclastic materials, including benthic foraminifera, crustal red algae, and red algae detritus, with a few volcanic gravel clasts. The content of volcanic rock gravels decreases towards the top, which may be attributed to the reduction or disappearance of volcanic rocks in the provenance area due to the establishment of coral reef facies that cap the succession and dominate the overlying interval from a core depth of 885.6 m to the seafloor. Based on U-Pb dating, the cap-shaped red algae laminae at the facies transition were formed at 24.6 ± 1.1 Ma, dating the earliest initiation of reef building to end-Oligocene times. The emergence of gravity flow sediments during the early Oligocene may be attributed to the onset of the Antarctic glaciation and concurrent regional tectonic activities that led to a shallowing of the sea. The origin of debris with high roundness characteristics is believed to be associated with transportation along the slope triggered by tectonic and climatic events, resulting in the formation of debris flow deposits. Studying the gravity flow facies at the Meiji Reef system is crucial for determining the onset of reef-building and helps establish the relationship between gravity flow deposition, paleoclimate, and regional tectonic evolution in the South China Sea. • On the volcanic basement of Well NK1 in the South China Sea, unique gravity flow deposits with high roundness and low sorting are developed. • U-Pb dating of carbonate rocks shows that the gravity flow deposits were formed during the Oligocene with an age of 26.1–23.5 Ma, which are debris flow deposition. • U-Pb dating of carbonate rocks shows that the age of the cap-shaped red algae laminae around the gravels are formed at 24.6 ± 1.1 Ma, which is the earliest initiation time of reef building in the Cenozoic. • The emergence of gravity flow deposits during the Early Oligocene is an important sedimentary response to a series of events, including global ice age events, regional tectonic activities, and fluctuations in sea level. [ABSTRACT FROM AUTHOR]

The Jurassic magmatic and volcanic rocks are widespread along the west central Lhasa subterrane. However, the petrogenesis of these rocks is poorly understood because of lacking high-quality geochronology and geochemical data. Here, we present new zircon U-Pb age and Hf isotopic data, whole-rock geochemical and Sr-Nd-Pb isotopic data for the Songduole and Qiangnong plutons in Geji area. LA-ICP-MS dating of zircon yield crystallization ages of 172.1 ± 1.9 and 155.9 ± 1.2 Ma for the Songduole and Qiangnong plutons, respectively. Geochemically, Songduole and Qiangnong granodiorite are characterized by high MgO (2.63-3.49 wt%), high Mg# (49-50), and low TiO2 (0.48-0.57 wt%). Besides, all rocks show metaluminous, calc-alkaline signatures, with strong depletion of Nb, Ta, and Ti, enrichment of large-ion lithophile (e.g. Rb, Th, K), and a negative correlation between SiO2 and P2O5. All these features are indicative of arc-related I-type magmatism. Five samples from the Songduole granodiorite have whole rock (87Sr/86Sr)i of 0.71207-0.71257, εNd(t) values of −15.1 to −13.9, zircon εHf(t) values of −17.4 to −10.5, (206Pb/204Pb)t ratios of 18.402-18.854, (207Pb/204Pb)t ratios of 15.660-15.736, and (208Pb/204Pb)t ratios of 38.436-39.208. Samples from the Qiangnong granodiorite have (87Sr/86Sr)i of 0.71230-0.71252, εNd(t) values of −15.1 to −14.2, zircon εHf(t) values of −12.6 to −6.4, (206Pb/204Pb)t ratios of 18.688-18.766, (207Pb/204Pb)t ratios of 15.696-15.717, and (208Pb/204Pb)t ratios of 38.546-39.083. These geochemical signatures indicate that the two plutons most likely originated from partial melting of the ancient Lhasa lower crust with obvious inputs of mantle-derived melts. Combined with regional geology, our results indicate that the Jurassic magmatism in the west central Lhasa subterrane most likely resulted from the southward subduction of the Bangong Ocean lithosphere beneath the central Lhasa terrane. [ABSTRACT FROM AUTHOR]

This study deals with arc-type and subsequent bimodal volcanic rocks interbedded with (late) Cretaceous sedimentary formations near Gaize, central Tibet that shed new light on the Tethyan evolution along the Bangong-Nujiang suture. Unit I consists of trachyandesites interbedded with fine-grained sandstone, slate, and limestone. Zircon dating on a trachyandesite sample yields a 206 Pb/ 238 U age of 99 ± 1 Ma. The trachyandesites are characterized by strong enrichment in LILE but depletion in HFSE, low zircon saturation temperatures (T Zr = 642–727 °C), and high oxygen fugacity (∆ FMQ = − 0.67–0.73), indicating their arc affinities. Unit II comprises a bimodal basalt-rhyolite suite interbedded with coarse-grained sandstone and conglomerate. Zircon dating on two rhyolitic samples yield 206 Pb/ 238 U ages of 97.1–87.0 Ma. In contrast with Unit I trachyandesites, Unit II basalts and rhyolites exhibit OIB-like trace element patterns, high temperatures ( T = 1298–1379 °C for basalts, T Zr = 855–930 °C for rhyolites), and low oxygen fugacity (∆ FMQ = − 3.37 ~ − 0.43), suggesting that Unit II bimodal volcanic rocks probably formed in an extensional setting. The Sr-Nd isotopes of both Unit I ( 87 Sr/ 86 Sr i = 0.7052–0.7074, εNd(t) = − 2.21–1.02) and Unit II ( 87 Sr/ 86 Sr i = 0.7057–0.7098, εNd(t) = − 3.22–0.88) rocks are similar to mantle-wedge-derived volcanic rocks within the southern Qiangtang block. The parental magma of Unit I trachyandesites was formed by fluid induced melting of the mantle wedge above the subducted Bangong-Nujiang Tethyan slab, and contaminated by crustal materials during MASH process within a deep crustal hot zone; and Unit II bimodal volcanic rocks were derived by melting of upwelling asthenosphere and a mildly metasomatized mantle wedge during the Lhasa-Qiangtang collision. We propose that the transition from the Bangong-Nujiang Tethyan subduction to the Lhasa-Qiangtang collision occurred during the Late Cretaceous in central Tibet. [ABSTRACT FROM AUTHOR]

Rheological differences may lead to discrepant thinning order of the crust and lithospheric mantle when the continent is passively stretched. The crust breaks up earlier than the lithospheric mantle under a low stretching rate when the lower crust is strong; otherwise, the lithospheric mantle may break up earlier than the crust. Here we report a new high-resolution wide-angle seismic velocity model from ocean bottom seismometers (OBS) across the northern continental margin of the South China Sea (SCS). Significant crustal structure and deformation style discrepancies are observed from the proximal to distal margin of the northern SCS. High-velocity lower crust (HVLC) up to 10 km in thickness was observed below the Yunli uplift and the oceanward margin, indicating magmatic underplating, while nearly no HVLC was found below the highly extended landward sags. Seismic data showed a low ratio of upper/lower crustal thickness below the underplating area, presumably caused by magmatic accretion and lower crustal flow. Largely seaward-dipping faults with small vertical offsets were observed on the seaward side of the HVLC. The OBS and multichannel seismic data favor a model with the lithospheric mantle breaking up prior to the crust at the SCS. Based on the observed stratigraphic relationship and volcanic rocks dating results, we conclude that erosion of the sedimentary strata is synchronous to the magmatic underplating, which indicates that lithospheric mantle breakup occurred at ∼43–38 Ma. The northern central SCS breakup occurred in two steps, in which the lithospheric mantle broke up ∼10 Myr earlier than the crust (at ∼31–29 Ma). • A newly high-resolution wide-angle seismic velocity model across the northern margin of the South China Sea was reported. • Nearly 10 km-thick high-velocity lower crust was identified below the distal margin. • We proposed a model that the lithospheric mantle broken up prior to the crust at the SCS. [ABSTRACT FROM AUTHOR]

A combined rock-magnetic and paleomagnetic researches were carried out on some pyroclastic rocks and pre-Columbian pottery fragments associated to the Colima Volcanic Complex. A full battery of magnetic measurements was performed in order to estimate magnetic mineralogy and to determine both the directions and absolute intensities of the characteristic remanent magnetization. In this paper we report absolute age determination using magnetic dating for two volcanic eruptions associated to the Colima volcano (Volcán de Fuego). The geomagnetic field prediction model SHA. DIF.14K, based on full vector (declination, inclination and absolute paleointensity), was used for first time in pyroclastic flows deposits and intercalated potteries. The results obtained agree reasonably well with available radiometric (C14) data, them provide a more accurate age determination of the cooling moment for the lava flows. [ABSTRACT FROM AUTHOR]

The Takht-e-Gonbad, or simply Takht, copper deposit in the Kerman belt in southern section of the Cenozoic Urumieh-Dokhtar magmatic arc (UDMA) of Iran is spatially associated with late Oligocene shallow granodiorite porphyries intruded into pyroclastic rocks. Hypogene ore minerals include pyrite, chalcopyrite, magnetite and rare molybdenite and bornite, occurring as veinlets and disseminations mostly in the pyroclastic rocks. Hydrothermal alteration is marked by an extensive phyllic assemblage and irregular zones of propylitic and calc-silicate assemblages with no consistent zoning patterns. The various igneous rocks at Takht area are calc-alkaline to shoshonitic, and are distinguished by enrichments in LILEs relative to HFSEs, and LREEs relative to HREEs (14.41 ≥ La N /Yb N ≥ 2.85). These features suggest a subduction-related setting for the rocks and the associated copper deposit in the area. A late Oligocene age is obtained for the main Takht granodiorite porphyry and the granodiorite batholith (24.2 Ma and 25.0 Ma, respectively, zircon U-Pb). The age of the main Takht porphyry is distinctly older than the middle-late Miocene ages reported for most porphyry Cu deposits occurring to the north of the Takht deposit, including the world-class Sarcheshmeh and Meiduk. Three distinct types of fluid inclusions are identified: F1: L + V, V < 50%; F2: L + V + S; and F3: V + L, L < 10%. The F2 type inclusions with T h values ranging between 340 °C and >600 °C and salinities from 55 to 68 wt% NaCl eq best represent characteristics of the mineralizing hydrothermal fluids. The δ 34 S values are measured for pyrite and chalcopyrite; the calculated δ 34 S values for ore fluids fall in the range +4.4 to +7.4‰, slightly heavier than those reported from most other porphyry copper deposits (PCDs) elsewhere. The shift towards more positive values could be attributed to contribution of isotopically heavy sulfur from crustal sources or involvement of marine sulfate in the magma source. The δ 18 O SMOW values for quartz veins in the phyllic assemblage define a narrow range, between +7.6 and +8.1‰. The calculated δ 18 O values for ore fluids range between +6.0 and +6.6‰ suggesting a magmatic source for the hydrothermal fluids. In spite of clear similarities with classic PCDs, in terms of ore mineralogy, ore texture/structure, and ore fluid characteristics, the Takht deposit stands distinct from most known PCDs in the Kerman belt in that most mineralization occurs in the pyroclastic host rocks and in close association with phyllic alteration. [ABSTRACT FROM AUTHOR]

To constrain the paleolatitude evolution and orientation of the Lhasa terrane in the Cretaceous, we performed a paleomagnetic study on the Qushenla Formation volcanic rocks dated to ∼103 Ma near Nima County in the central part of the Lhasa terrane. Rock magnetism and magnetic petrography indicate that the primary magnetic carriers are magnetite and hematite that formed during initial cooling. A high-temperature component was isolated between ∼500 °C and ∼680 °C. The site-mean direction passes the fold tests at a 99% confidence level, yielding a mean inclination of 35.4° for 18 sites after tilt correction. These results suggest that the volcanic rocks recorded a primary remnant magnetization, revealing a paleolatitude of ∼17.9°N at ∼103 Ma for the reference point (29.0°N, 87.5°E) at the southern margin of the Lhasa terrane. Together with previous paleomagnetic results from the Lhasa terrane, our results indicate WNW–ESE elongation of the stationary Lhasa terrane at ∼6.7–19°N during the Cretaceous. The paleolatitude evolution of the Lhasa terrane during the Jurassic–Cretaceous suggests that the Lhasa–Qiangtang collision occured at ∼155–132 Ma. • The Qushenla Formation volcanic rocks have recorded the prefold magnetization. • The southern margin of the Lhasa terrane was oriented in a WNW-ESE direction at ∼6.7–19.0°N during the Cretaceous. • The Lhasa-Qiangtang collision occurred at ∼155–132 Ma. [ABSTRACT FROM AUTHOR]

Flores represents a unique insular environment with an extensive record of Pleistocene fossil remains and stone artefacts. In the So'a Basin of central Flores these include endemic Stegodon , Komodo dragons, giant tortoises, rats, birds and hominins, and lithic artefacts that can be traced back to at least one million years ago (1 Ma). This comprehensive review presents important new data regarding the dating and faunal sequence of the So'a Basin, including the site of Mata Menge where Homo floresiensis -like fossils dating to approximately 0.7 Ma were discovered in 2014. By chemical fingerprinting key silicic tephra originating from local and distal eruptive sources we have now established basin-wide tephrostratigraphic correlations, and, together with new numerical ages, present an update of the chronostratigraphy of the So'a Basin, with major implications for the faunal sequence. These results show that a giant tortoise and the diminutive proboscidean Stegodon sondaari last occurred at the site of Tangi Talo ∼1.3 Ma, and not 0.9 Ma as previously thought. We also present new data suggesting that the disappearance of giant tortoise and S. sondaari from the sedimentary record occurred before , and/or was coincident with , the earliest hominin arrival, as evidenced by the first records of lithic artefacts occurring directly below the 1 Ma Wolo Sege Tephra. Artefacts become common in the younger layers, associated with a distinct fauna characterized by the medium-sized Stegodon florensis and giant rat Hooijeromys nusatenggara. Furthermore, we describe a newly discovered terrace fill, which extends the faunal record of Stegodon in the So'a Basin to the Late Pleistocene. Our evidence also suggests that the paleoenvironment of the So'a Basin became drier around the time of the observed faunal transition and arrival of hominins on the island, which could be related to an astronomically-forced climate response at the onset of the Mid-Pleistocene Transition (MPT; ∼1.25 Ma) leading to increased aridity and monsoonal intensity. • The ancestral population of Homo floresiensis probably arrived on Flores after 1.3 Ma but certainly before 1 Ma. • The oldest megafauna on Flores, including Komodo dragon, diminutive Stegodon and giant tortoise, was present on Flores by 1.4 Ma. • A fauna turnover coincided with the Mid-Pleistocene Transition and a shift from a forested to a grassland dominated biotope in the So'a Basin. • Despite major volcanic eruptions and climate oscillations a stable fauna appears to have persisted until the arrival of humans at 46 ka. • Chemical fingerprinting of distal tephra layers in the So'a Basin has the potential for regional tephrostratigraphic correlations. [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]

Volcanic and plutonic rocks exposed in the Latir volcanic field, northern New Mexico, provide an opportunity to examine the timing of caldera-related magmatic processes. Fifty-five mineral separates were dated using the 40Ar/39Ar method. Volcanism began at 28.5 Ma and ended at 22.7 Ma, climaxing with the eruption of the high-SiO2 peralkaline Amalia Tuff from the Questa caldera at 25.39 ± 0.04 Ma. Following caldera collapse, four resurgent plutons were emplaced between 25.4 and 25.0 Ma. Previous geochemical studies suggested that the peralkaline phases of two resurgent plutons and a peralkaline ring dike represent an intrusive equivalent to the Amalia Tuff. A new analysis of K-feldspar from the ring dike yielded a plateau age of 25.38 ± 0.04 Ma, supporting this previous conclusion. Only these small-volume peralkaline intrusions are temporally indistinguishable from syncaldera magmatism. The structurally lower resurgent plutons were emplaced ∼100--400 k.y. after the Amalia Tuff eruption and are therefore too young to be the non-erupted, more-mafic parts of Amalia Tuff magma chamber, implying that the Amalia Tuff magma was not generated via in situ differentiation in the upper crust. Following resurgent plutonism, five additional plutons were emplaced between 24.9 and 19.3 Ma along the southern caldera margin and south of the caldera. Thermochronology indicates that most plutons rapidly cooled following emplacement, but that some were subsequently reheated during protracted post-caldera pluton emplacement. The timing of pluton emplacement at the Questa caldera indicates that only small volumes of the intrusive rocks are the same age as the caldera eruption and that most of the intrusive rocks record postcaldera magmatic processes. [ABSTRACT FROM AUTHOR]

The characterization of five lithic raw materials identified in the Doncellas locality (Cochinoca Department, Jujuy) is presented, as well as the results of a comparative petrographic analysis of thin sections from eight rock samples. Five of the samples were taken from the primary sources, while the three others are from the archaeological sites: Doncellas (or Agua Caliente de Rachaite), Sayate and Sorcuyo (or Pueblo Viejo de Tucute). The results indicate that while there are two types of volcanic rocks represented (andesites and basalts), the observed variability allows the discrimination of four different groups based on their petrographic characteristics. Starting from the characterization of primary sources, we seek to understand and explain the variability of the artifact assemblages and the supply of local resources. [ABSTRACT FROM AUTHOR]

The article offers information on the determination of the age of granitoid plutons in Chauna zone of Verkhoyansk mountains in Chukotka, Russia. It mentions that granitoids in the said zone are composed of batholiths with an area of up to 2000 km2. It cites a dating of zircons from granitoids of two plutons Velitkenai and Moltykan in North Chukotka done though the process of elongation of zircons. It states the age of the samples estimated at 107.0 ± 1.0 Ma for Moltykan pluton and 105.3 ± 1.2 Ma for Velitkenai pluton.

Silicic volcanism at c. 168 Ma has been identified previously on the Antarctic Peninsula, and the Mapple Formation, which includes those volcanic rocks, has been defined and documented from one area of the east coast of Graham Land. Based on age and geochemical criteria, correlations have been made to the extensive Chon Aike Province of South America, which has been demonstrated to be one of the largest silicic volcanic provinces in the world. Rhyolitic and intermediate composition volcanic successions from six separate localities on the east coast of the Antarctic Peninsula are described here and are confirmed as correlatives of the Mapple Formation, based on newly acquired geochronology and field observations. They are dominantly rhyolitic crystal tuffs and/or ignimbrites with ages in the interval 162-168 Ma, overlapping with the age of the Mapple Formation (167-171 Ma) at the type locality. Andesitic agglomerates are also described, which are included in the same event and demonstrate the occurrence of rare intermediate volcanism, which is also seen in the Chon Aike Province. A new group, the Graham Land Volcanic Group, is defined here, and criteria are established which allow the separation of some volcanic successions out of the previously defined Antarctic Peninsula Volcanic Group, which takes no account of tectonic setting, eruption age or geochemistry. [ABSTRACT FROM AUTHOR]

Abstract: Cosmogenic 21Ne was utilised to determine exposure ages of young subaerial basaltic lava flows from the Newer Volcanic Province, western Victoria, Australia. The ages (36–53ka) determined from co-existing cosmogenic 21Ne and 3He in olivines separated from basalts are consistent within analytical uncertainties with ages previously determined by cosmogenic 36Cl exposure dating. This paper illustrates the potential of cosmogenic neon exposure ages in studying the eruption, surface morphology, and erosion history of young volcanic rocks, which are difficult to date using other conventional methods, such as K-Ar or 40Ar/39Ar dating. The present study demonstrates that combined cosmogenic 3He and 21Ne dating, specifically measured cosmogenic 3He/21Ne ratios, on the same samples, is powerful for evaluating the validity of calculated cosmogenic 3He and 21Ne surface exposure ages. [Copyright &y& Elsevier]

In West Liaoning-North Hebei, the Tiaojishan Formation volcanic rocks are developed and many isotopic ages with a very wide age range were obtained in them as well. These different ages have direct influence on confirming and interpreting the related geological issues in West Liaoning-North Hebei and have restricted the study of geological problems related. In the field, the Tiaojishan Formation and the overlying Tuchengzi Formation are in parallel unconformable contact. By systematical sampling and dating of LA-ICP-MS in the Tiaojishan Formation (especially in the upper part) in Luanping, Chengde basins of Northern Hebei Province and in Lingyuan, Jinlingsi-Yangshan basins of Western Liaoning Province, combined with the dating data obtained by previous researchers, results indicate that the upper boundary age of the Tiaojishan Formation is from 156 to 153 Ma, and the lower boundary age of the Tiaojishan Formation is about 165 Ma, the Tiaojishan Formation was formed from the late of middle Jurassic to late Jurassic Epoch. [ABSTRACT FROM AUTHOR]

Abstract: The geological cross section of the Zhangjiakou Formation is originally established in the Zhangjiakou region. From the dating of four samples from typical cross sections of the Zhangjiakou Formation in the Zhangjiakou region, the age range is found to be mainly from 143.0±3.7Myr to 136.1±1.4Myr, which almost equals the sum of the age range of the Zhangjiakou Formation (136–135Myr) and that of the Tuchengzi Formation (143–136Myr) in the Luanping region, and the Zhangjiakou and Tuchengzi Formations in Zhangjiakou-Luanping of North Hebei are found to be conformable. The evidence above indicates that the Zhangjiakou and Tuchengzi Formations, which are distributed in Zhangjiakou-Luanping, were formed under the same geological setting. Volcanic activity was earlier and longer (143–136Myr) in the western region (Zhangjiakou) than that in the eastern region (Luanping). The “Zhangjiakou Formation” in Chengde and Pingquan formed in the period in which the Yixian Formation in West Liaoning formed, so should be renamed the Yixian Formation. [Copyright &y& Elsevier]

This study presents the results of radiometric K-Ar measurements on separated amphiboles from the andesite of the Dubie complex dyke. The data obtained cover the period of (291.3 ± 6.4) Ma, which corresponds to Carboniferous-Permian transition. The age is contemporaneous to the rhyodacitic and basaltoid volcanism of the Krakow region. [ABSTRACT FROM AUTHOR]

An attempt was made to use CO2 laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the fourteen heating steps, ten define a good inverse isochron (MSWD = 1.4) with an age of 32.2 ± 7.1 ka (2 σ).The inverse isochron also shows that the initial argon isotopic ratio is 297.1±2.0 (2σ) which is the same as the atmospheric argon at the 2σ error level. Study indicates that it is a useful means to date young volcanic groundmass with low K content by using CO2 laser step-heating 40Ar/39Ar method. [ABSTRACT FROM AUTHOR]

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight 40Ar/39Ar ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 ± 0.6 Ma, which is 12–17 m.y. younger than a widely cited age of 55–50 Ma. Three main pulses of arc-wide magmatism occurred at 38–29, 16–11, and 6–0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude. [ABSTRACT FROM AUTHOR]

The natural hydration of obsidian was first proposed as a dating technique for young geological and archaeological specimens by Friedman and Smith (1960), who noted that the thickness of the hydrated layer on obsidian artifacts increases with time. This approach is, however, sensitive to temperature and humidity under earth-surface conditions. This has made obsidian hydration dating more difficult, but potentially provides a unique tool for paleoclimatic reconstructions. In this paper we present the first successful application of this approach, based on combining laboratory-based experimental calibrations with archaeological samples from the Chalco site in the Basin of Mexico, dated using stratigraphically correlated 14C results and measuring hydration depths by secondary ion mass spectrometry. The resultant data suggest, first, that this approach is viable, even given the existing uncertainties, and that a cooling trend occurred in the Basin of Mexico over the past 1450 yr, a result corroborated by other paleoclimatic data. [ABSTRACT FROM AUTHOR]

Abstract: The potential benefit of red thermoluminescence (RTL) dating of quartz is related to the signal''s stability over time scales of 100ka to 1Ma. RTL dating has been tested on volcanic quartz grains in a late Pleistocene pyroclastic flow which erupted from the Toya caldera in Hokkaido, Japan. New TL equipment installed with a small X-ray irradiator facilitated using the regenerative dose method for single grain (or aliquot) for RTL dating. A suitable combination of filters and photomultiplier gave effective RTL readouts while eliminating the black body emission of the heating device. Preliminary examination of the output showed that the new equipment provided RTL signals even from single quartz grains. The RTL signal did not show any sensitivity changes after 10 cycles of reproducibility, making it suitable for a construction of a regenerative dose curve for each grain and aliquot, with no correction for sensitivity changes. The ages resulting from the regenerative dose method are in good agreement with that of a multiple aliquot slide method and approximately concordant with ages estimates based on previous geological evidence. [Copyright &y& Elsevier]

Abstract: We present the results of the first archeometric study of some Roman millstones and mortars made of volcanic rocks found at the Libyan archeological sites of Leptis Magna and Cyrene. Two millstones from the first site dating from the III century AD and one from the second, also dating from the II–III centuries AD, were found to made from leucite–phonolite lavas from the quarrying area near Orvieto, in the Vulsini Volcanic District (Roman Volcanic Province). These findings provide further confirmation of the central role played by Orvieto in the Roman production and export of millstones. Mugearite lavas from Etna, transitional basalt from the island of Pantelleria, alkaline basalt, probably from Al Haruj-Al Aswad (Central Libya), and high-K calc-alkaline andesite of uncertain geological origin were the other rocks used for the millstones investigated in this study. This evidence clearly indicates a trade in millstones between Tripolitania, Cyrenaica and Central-South Italy. [Copyright &y& Elsevier]

The Ericiyes Basin is a trans-tensional basin situated 20 km north of the regional Ecemiş Fault Zone. Recently it has been hypothesized that faulting within the Erciyes Basin links with the Ecemiş Fault Zone further south as part of a regional Central Anatolian Fault Zone. New 40Ar/39Ar dating of volcanic and volcaniclastic rocks adjacent to faults, both along the margins and in the centre of the Erciyes Basin, constrains the timing of basin inception and later faulting. Extensional faulting occurred along the eastern and western margins of the basin during the Early Messinian (latest Miocene). Sinistral and minor normal faulting were active along the axis of the basin during the early Pleistocene. These fault timings are similar to those inferred for the Ecemiş Fault Zone further south, and support the hypothesis that faulting within the Erciyes Basin and the Ecemiş Fault Zone are indeed linked. Terra Nova, 00, 1–9, 2004 [ABSTRACT FROM AUTHOR]

Volcanic ash beds from shallow- to deep-water facies strata of the Sinian-Early Cambrian (Meishucunian) on the Yangtze Platform consist of bentonites and tuffites which are readily recognized in the field by their physical features and confirmed by geochemical analyses. Geochemistry suggests that the volcanic ash beds in Meishucunian time are rhyolite and rhyodacite while those in the Qiongzhusian and Sinian are andesite and trachyandesite. The ash beds in the time-equivalent strata, even in different areas display rather similar geochemical features, whereas the ash beds in different strata even in the same areas show large chemical difference. The results suggest that these ash beds can be used for intra- and extra-basinal correlations of the Sinian-Early Cambrian interval on the Yangtze Plat-form. Additionally, these ash beds suggest high potentials for further U-Pb dating strategies. [ABSTRACT FROM AUTHOR]

Besides older granites and gneisses, the Thu¨ringer Wald in central Europe consists predominantly of altered Permian volcanic and pyroclastic rocks The alteration history of these volcanic rocks was investigated by K–Ar, 39ArK–40Ar, Rb–Sr, δ18O, δD and X-ray studies on feldspar and biotite separates. Alteration is accompanied by increases in the triclinicity and δ18O values (+14‰ to +19‰) of alkali feldspars, an increase in the δ18O values of low-potassium biotites (>+10‰) and in sericitized plagioclases (+12‰ to +19‰) and also the generation of apparent Mesozoic isotopic ages. In contrast, volcanic biotites with primary magmatic δ18O signatures still yield Early Permian ages, and biotites of the granitic basement give Carboniferous ages.The observed isotope changes of the volcanic feldspars indicate episodic fluid exchange with meteoric and formation brines (δ18O≤+5‰) at low temperatures (200 to 100 °C) during tectonic fracturing of the basement. As a result, sanidines were replaced by porous microclines which were subject to isotope exchange and Ar loss. Feldspars and biotites of the granitic wall rocks seem to be less affected by alteration processes, which may be due to their lower permeabilities.Plateau-shaped 40Ar/39Ar age spectra of volcanic alkali feldspar indicate complete resetting of the K–Ar system, whereas staircase spectra point to partial retention or repeated Ar loss. Because the feldspars represent mixtures of microcline and sanidine remnants, the apparent ages (260 to 160 Ma) represent maximum ages for the time of replacement. Rb–Sr feldspar model ages (180 Ma or 230 to 170 Ma, respectively) support the K–Ar data. The minimal 40Ar/39Ar spectra ages are concordant with both K–Ar and 40Ar/39Ar sericite model ages (160 to 150 Ma) of sericitized plagioclases. The model ages favour Jurassic episodic fluid reactions with alkali feldspars and plagioclases in the Thu¨ringer Wald. [Copyright &y& Elsevier]

Discusses the Zircon SHRIMP uranium-lead dating for the Cangshuipu volcanic rocks and its implications for the lower boundary age of the Nanhua strata in South China. Occurrence of the continental volcanic rocks and volcaniclastic sedimentary conglomerates of the Cangshuipu Formulation in Yiyang of Hunan Province; Configuration of the strata in the Yiyang region; Representation of the lower boundary with a younger age than previously reported.

Presents information on a study which described the high-precision thermal ionization mass spectrometry dating of young volcanic rocks of Tengchong in Yunnan Province, China using U-series method. Information on the Tengchong volcanic field; Procedure for U and Th separation and purification.

The collision between the Qiangtang and Lhasa blocks is a critical factor in understanding the geodynamics of the central Tibetan Plateau. However, the process of the Qiangtang-Lhasa collision remains contentious. A direct way to study the history of the Qiangtang-Lhasa collision would be to determine the evolution of the paleolatitude positions of the Qiangtang and Lhasa blocks during the Jurassic-Cretaceous. In this study, we present a combined paleomagnetic and geochronological study of the Early Cretaceous volcanic rocks dated at ~120–115 Ma in the western Qiangtang block. Stepwise thermal demagnetization succeeded in isolating the high-temperature characteristic directions of the samples. The tilt-corrected mean direction of the 16 sampling sites was Ds = 60.9°, Is = 45.9° with ɑ 95 = 4.4°, which indicates that the Qiangtang block was situated at 27.6 ± 5.0°N during the Early Cretaceous (~120–115 Ma) (reference point: 32.9°N, 83.5°E). Our new data combined with previous reliable Cretaceous paleomagnetic results for the Lhasa block indicate that the collision between the western parts of the Qiangtang and Lhasa blocks may occur later than 115 Ma. The extent of the western segment of the Bangong-Nujiang Tethys Ocean was ~825 ± 600 km (7.5° ± 5.5° in latitude) at 120–115 Ma. • The southwestern margin of the Qiangtang block was situated at 27.6 ± 5.0°N at 120–115 Ma. • The extent of the western segment of the Bangong-Nujiang Tethys Ocean was ~825 ± 600 km at 120–115 Ma. • The collision between the western parts of the Qiangtang and Lhasa blocks occurred later than 115 Ma. [ABSTRACT FROM AUTHOR]

• The Malmberget IOA deposit is hosted by volcanic rocks intruded by various dikes. • The volcanic rocks are dated at 1885 ± 6 Ma and 1881 ± 6 Ma respectively. • Granitic and aplitic dikes are dated at c. 1790 Ma. • A metamorphic overprint event in the Gällivare area is dated at 1775 ± 6 Ma. • Geochemical signatures indicate generation in a volcanic arc environment. The northern Norrbotten region in Sweden hosts abundant iron-oxide apatite (IOA) deposits including Kiirunavaara, the type locality for Kiruna-type deposits, and Malmberget. Felsic and intermediate metavolcanic rocks hosting the Malmberget IOA deposit contain oscillatory zoned zircon which yield magmatic U-Pb SIMS ages of 1885 ± 6 Ma and 1881 ± 6 Ma, respectively. Metamorphic rims on zircon from these rocks yield 1797 ± 7 Ma and 1775 ± 6 Ma, respectively, and record the age of the latest Svecofennian regional metamorphic event in the Gällivare area, tentatively interpreted as regional contact metamorphism. Two granite dikes that cut the ore yield U-Pb zircon emplacement ages of 1790 ± 6 Ma and 1791 ± 7 Ma, respectively, overlapping with the metamorphic overgrowths, and set a lower age limit for ore formation in the Malmberget IOA deposit. Rocks hosting the Malmberget IOA deposit have an alkalic to alkali-calcic affinity with a geochemical signature that favors a continental-arc, transitional to extensional setting. These rocks are suggested to have been generated in a back-arc region, in response to subduction beneath the craton margin retreating to the SW or W. The obtained ages and geochemical signatures of these rocks coincide well with the regionally defined Kiirunavaara group rocks, hosting several other IOA deposits in northern Sweden. [ABSTRACT FROM AUTHOR]

• First dates for host rocks of largest geothermal field in central TVZ. • U-Pb and U-Th zircon ages range from 1.65 million to 60 thousand years. • The modern Wairakei-Tauhara geothermal system was probably initiated at ca. 60 ka. • Zircon ages plus new stratigraphy constrain major volcano-tectonic events. The central part of the Taupo Volcanic Zone (TVZ), New Zealand, is notable for the number (23) and size of its high-temperature geothermal systems. Of these, the Wairakei-Tauhara geothermal system collectively represents the largest commercially developed example. Natural thermal output prior to utilisation, i.e. before 1950 was ∼530 MW. Since 1950, drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework. Within this framework, the strata are dominantly rhyolitic pyroclastic rocks and lava bodies, with interbedded fluvial and lacustrine deposits. Until now, no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25.4 ka Oruanui ignimbrite, or to buried ignimbrite that is correlated with the 349 ka Whakamaru Group. This paper presents 33 suites of U-Pb age data and a limited number of reconnaissance U-Th disequilibrium age data from magmatic zircon crystals (zircons) to date the rocks of the Wairakei-Tauhara system. The oldest units dated are intermediate lavas from ca. 1.7 Ma, while the youngest locally vented eruptives of Tauhara dacite are tentatively dated to ca. 60 ka. Placing the age data within the established drillhole record allows for the recognition of major volcano-tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the Wairakei-Tauhara area. In contrast to the Rotokawa and Ngatamariki geothermal systems 10–20 km to the northeast, the Wairakei-Tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka, leading to major faulting and abrupt variations in thicknesses of units. The faulting and deposit thickness variations established in these two events remain major controls on present-day permeability of the geothermal system. The new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity. However, from zircon age data from the rhyolite-andesite mixed Tauhara dacite, we propose that the current geothermal system was initiated at ca. 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since. [ABSTRACT FROM AUTHOR]

The article discusses the exposure of Earth's older rocks in comparison with younger rocks and the use of geologic maps to gain information on the distribution of rocks and deposits. Topics include an overview of a study by researchers Wilkinson and colleagues who separate volcanic and sedimentary rocks from those that formed in the crust known as metamorphic and intrusive rocks. Statistics related to the coverage of old versus young rocks are presented.