Proto-Tethys magmatic evolution along northern Gondwana: Insights from Late Silurian–Middle Devonian A-type magmatism, East Kunlun Orogen, Northern Tibetan Plateau, China

The East Kunlun Orogen records the geological evolutions of the Neoproterozoic – Early Paleozoic Proto-Tethyan Ocean and Late Paleozoic–Mesozoic Paleo-Tethys Ocean along northern Gondwana. However, the late-stage evolution of the Proto-Tethyan Ocean and the configuration of peri-Gondwana microcontinents during the Silurian – Devonian is under debate. Here we report new geochronological and geochemical data of A-type granites from the western Wulonggou and the eastern Gouli areas in the East Kunlun Orogen to deepen our understanding of these problems. Zircon LA-ICP-MS UPb data reveal that the Danshuigou monzogranite and Shenshuitan syenogranite from the western Wulonggou area were emplaced simultaneously at 418 ± 3 Ma, while the Niantang syenogranite from the eastern Gouli area was emplaced at 403 ± 2 Ma. All these rocks display high-K calcic-alkalic to shoshonitic and metaluminous to slight peraluminous signatures, with relatively low CaO, Al2O3, MgO and Sr, and high FeOt/MgO, Ga/Al, Zr, and Nb, indicating their A-type affinity. Their moderate whole-rock εNd(t) (−5.3 to −0.6) and zircon εHf(t) (−6.3–6.4) are different from those of depleted mantle and old basement rocks, but similar to those of the Ordovician–Silurian granitoids in the East Kunlun Orogen. These chemical signatures, together with the anhydrous, low-pressure and high-temperature characteristics of the magmas, indicate that partial melting of the Ordovician–Silurian granitoids generated these A-type granites. Regionally, these A-type granites and previously reported A-type granites in the East Kunlun Orogen compose a Late Silurian – Middle Devonian A-type granite belt. This belt, together with the regionally coeval molasse formation and mafic-ultramafic rocks, indicate a post-collisional extensional regime for the East Kunlun Orogen during the Late Silurian – Middle Devonian. Given that extensive contemporaneous post-collision-related magmatic rocks have also been revealed in the neighboring West Kunlun, Altyn, Qilian and Qinling blocks/terranes, we contend that the Neoproterozoic – Early Paleozoic Proto-Tethyan Ocean that separated these blocks/terranes from Gondwana had closed by the Late Silurian – Middle Devonian, which]resulted in the re-welding of the above blocks/terranes to northern Gondwana or Gondwana-derived microcontinents

Genesis of the Wutuogou Ag-Pb-Zn deposit in the Eastern Kunlun Orogenic Belt, NW China: Constraints from calcite U-Pb geochronology, mineral chemistry, and in-situ sulfur isotopes

The Wutuogou Ag-Pb-Zn deposit, a newly discovered vein-type deposit, is located in the Eastern Kunlun Orogenic Belt (EKOB), northwestern China. The vein-type Ag-Pb-Zn ore bodies are hosted in Middle Triassic granodiorite and monzogranite and are characterized by high-grade Ag, Pb, and Zn (average Ag: 293 g/t, Pb: 3.00 %, Zn: 2.85 %). Three paragenetic stages have been recognized: quartz + pyrite (Py-1) + arsenopyrite (stage I), pyrite (Py-2) + sphalerite + chalcopyrite + tetrahedrite + quartz (substage II-1), galena + pyrargyrite + freibergite + freieslebenite + quartz + calcite (substage II-2), and quartz + calcite (stage III). Except for Ag-bearing minerals (pyrargyrite, freibergite, and freieslebenite), invisible silver is also present in pyrite (1.91–165 ppm), sphalerite (3.86–8806 ppm), and galena (up to 0.21 wt%). The calcite is closely associated with sulfides in substage II-2 and yields a U-Pb age of 210 ± 7 Ma (MSWD = 2.7), which represents the ore-forming age (lower limit). Py-1 displays higher As contents and lower Co contents than those of Py-2, indicating a decrease in temperature from stage I to stage II. In addition, the Fe/Zn mass ratios (0.025–0.075) of sphalerite estimate the fluid temperature for substage II-1 of 246–284 ◦C, whereas the Ag/(Ag + Cu) and Zn/(Zn + Fe) mole ratios of freibergite estimate the fluid temperature for substage II-2 of 140–270 ◦C, further indicating the decrease of temperature from stage I through substage II-1 to substage II-2. Mineral assemblages of pyrite-chalcopyrite-tetrahedrite in substage II-1 and Ag-sulfosalts in substage II-2 suggest a decrease in sulfur fugacity (fS2). Both the decrease in fS2 and cooling of the mineralizing fluids facilitate silver precipitation. The heterogeneous compositions of the freibergite and the Ag zonation in sphalerite (Sp-1) resulted from retrograde solid-state reactions that redistributed Ag through microscale exsolution. The δ34S values (+5.49 to +7.78 ‰) of the sulfides and the low Zn/Cd ratios (107–195) of sphalerite indicate a felsic magma source for the ore-forming materials. Therefore, we concluded that the Wutuogou Ag-Pb-Zn deposit corresponds to a medium- to low-temperature magmatic-hydrothermal deposit associated with Late Triassic magmatism in the Eastern Kunlun Orogenic Belt (EKOB)Genesis of the Wutuogou Ag-Pb-Zn deposit in the Eastern Kunlun Orogenic Belt, NW China: Constraints from calcite U-Pb geochronology, mineral chemistry, and in-situ sulfur isotopespublishedVersio

Geology, U-Pb geochronology and stable isotope geochemistry of the Heihaibei gold deposit in the southern part of the Eastern Kunlun Orogenic Belt, China : A granitic intrusion-related gold deposit?

The Heihaibei gold deposit is a newly discovered gold deposit in the southern part of the Eastern Kunlun Orogenic Belt. Its most distinctive features are that the gold mineralization is hosted in monzogranite, and that the presence of pre-ore (possibly syn-ore) monzogranite and post-ore gabbro allows to constrain the minerali-zation's formation age. Zircons from the monzogranites yield U-Pb ages of 454 +/- 3 Ma, while zircons separated from the gabbro dikes cutting the monzogranites and gold mineralized body yield U-Pb ages of 439 +/- 3 Ma, which is interpreted to be the minimum age of the Au mineralizing event. Combined with the regional geological background, we proposed that the Heihaibei Au mineralization occurred during the subduction stage of the Early Paleozoic Proto-Tethys ocean. The ore assemblage is dominated by pyrite, arsenopyrite and native gold. The hydrothermal alteration that has led to the peculiar enrichment of Au is not systematically distributed and displays no clear concentric zoning pattern. The main mineralization formed during three stages: the K-feldspar-quartz-pyrite (Py1)-arsenopyrite-sericite-epidote stage (I), the quartz-pyrite (Py2)-native gold-chlorite stage (II), and the quartz-carbonate stage (III). The main gold mineralization occurred during stage II. Fluid inclusion homogenization temperature and salinities decrease from stage I (Th., 268-412 C; W., 6.87-16.63 wt% NaCl equiv.) to stage II (Th., 183-288 C; W., 3.69-14.84 wt% NaCl equiv.). The 818O and 8D values (818OH2O = 4.9 to 9.7%o; 8DV-SMOW =-84.1%o to -81.1%o) of quartz samples from stage I and stage II are comparable to a magmatic-hydrothermal ore-forming fluid that possibly underwent fluid-rock interaction with the Nachitai Group metamorphic rocks during the early ore-forming stage. The relatively uniform 834S values (834SV-CDT = 7.7 to 8.5%o) are slightly elevated compared to magmatic 834S values, but could be derived from a magma if a significant crustal melt component is present. Moreover, the 834S values are within the S isotopic composition range of a granitic reservoir, suggesting that they are probably inherited from the Heihaibei monzogranites. The Pb and Hf isotope compositions imply a close genetic association between the gold mineralization and granitic magmatism, which are both the products of the mixing of crustal and mantle sources. The trace element compositions of pyrite provide additional evidence that the gold mineralization in the Heihaibei deposit was related to the magmatism. Compared with the typical characteristics of orogenic gold and intrusion-related gold systems (IRGS) deposits, the Heihaibei gold deposit may instead be classified as a granitic intrusion-related gold deposit.Peer reviewe

Major and trace-element zoning in metamorphic garnets and their metamorphic process implications

This article introduces some of the modern techniques for analyzing compositional zonings/profiles in metamorphic garnet, emphasizing the significance of two-dimensional composition X-ray mapping for such investigations. The compositional zonings/profiles in garnet can be divided into uniform-changed type and abrupt-changed type. The uniform-changed type is formed in equilibrium-controlled growth; the abrupt-changed type may be subdivided into step-changed subtype and spike-changed subtype, the forming mechanism of each is relatively complex. The step-changed subtype may be related to the processes such as poly-metamorphism, p-T changes with large magnitudes, and changes in metamorphic reactions or disturbance of crustal melting. As an example, the petrography, the major-and trace-element zoning and distribution patterns of REEs in the garnet from the Xiaomiao Group in the East Kunlun Mountains suggest two episodes of regional metamorphism.published_or_final_versio

Oceanic subduction to continental collision in the NE Proto-Tethys revealed by Early Paleozoic eclogites with high-T granulite facies overprinting in the East Kunlun orogenic belt, northern Tibet

The East Kunlun orogenic belt in the northern Tibetan Plateau records a long-term accretionary and collisional history in the northeastern Proto-Tethys Ocean, which is important for reconstructing the paleogeography of early Paleozoic East Asia. Here, we present an integrated study combining petrology, geochemistry, geochronology, and metamorphic pressure−temperature (P−T) data of newly found eclogites in the middle Nuomuhong segment of the East Kunlun orogenic belt

High- and Ultrahigh-Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China

The North Qaidam and South Altyn terranes extend approximately 1000 km across the northern Tibetan Plateau, and five localities preserve evidence of Early Paleozoic high-pressure (HP) or ultrahigh-pressure (UHP) metamorphism, including the presence of coesite, coesite pseudomorphs, and diamond. A review of the geology, petrology, and geochronology collected over the past 10 years since these localities were discovered supports a correlation of the North Qaidam and South Altyn terranes, offset 350-400 km across the Altyn Tagh fault. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 500 and 420 Ma; detailed geochronology from one locality supports a protracted (tens of m.y.) history of HP/UHP metamorphism. Rock associations and geochronology support a passive-margin origin for the protolith of the HP/UHP rocks, which received sediments from a Proterozoic-Late Archean source, and was intruded by Neoproterozoic granites derived from crustal melting. Included here is the post-print copy of this article. The final publication is available from Taylor & Francis via http:// www.tandfonline.com/doi/abs/10.2747/0020-6814.49.11.969

Expansion of the Tibetan Plateau during the Neogene

The appearance of detritus shed from mountain ranges along the northern margin of the Tibetan Plateau heralds the Cenozoic development of high topography. Current estimates of the age of the basal conglomerate in the Qaidam basin place this event in Paleocene-Eocene. Here we present new magnetostratigraphy and mammalian biostratigraphy that refine the onset of basin fill to â 1/425.5 Myr and reveal that sediment accumulated continuously until â 1/44.8 Myr. Sediment provenance implies a sustained source in the East Kunlun Shan throughout this time period. However, the appearance of detritus from the Qilian Shan at â 1/412 Myr suggests emergence of topography north of the Qaidam occurred during the late Miocene. Our results imply that deformation and mountain building significantly post-date Indo-Asian collision and challenge the suggestion that the extent of the plateau has remained constant through time. Rather, our results require expansion of high topography during the past 25 Myr

Geochemistry, 40Ar/39Ar geochronology, and geodynamic implications of Early Cretaceous basalts from the western Qinling orogenic belt, China

This work was financially supported by Chinese National Natural Science Foundation (Grants. 41421002, 41302176 and 41190074), National Basic Research Program of China (2016YFC0600303 and 2014CB440901) and Foundation of Shaanxi Educational committee (14JK1760). P.A. Cawood acknowledges support from Australian Research Council (Grant FL160100168).The Qinling-Dabie orogenic belt was formed by the collision of the North and South China Cratons during the Early Mesozoic and subsequently developed into an intracontinental tectonic process during late Mesozoic. Field investigations identified the presence of late Mesozoic basalts in the Duofutun and Hongqiang areas in the western Qinling orogenic belt. The petrogenesis of these basalts provides an important constraint on the late Mesozoic geodynamics of the orogen. The representative basaltic samples yield the 40Ar/39Ar plateau age of about 112 Ma. These samples belong to the alkaline series and have SiO2 ranging from 44.98 wt.% to 48.19 wt.%, Na2O+K2O from 3.44 wt.% to 5.44 wt.%, and MgO from 7.25 wt.% to 12.19 wt.%. They demonstrate the right-sloping chondrite-normalized REE patterns with negligible Eu anomalies (1.00-1.10) and PM-normalized patterns enriched in light rare earth element, large ion lithophile element and high field strength element, similar to those of OIB rocks. These samples additionally show an OIB-like Sr-Nd isotopic signature with εNd(t) values ranging from +6.13 to +10.15 and initial 87Sr/86Sr ratios from 0.7028 to 0.7039, respectively. These samples are geochemically subdivided into two groups. Group 1 is characterized by low Al2O3 and high TiO2 and P2O5 contents, as well as high La/Yb ratios (>20), being the product of the high-pressure garnet fractionation from the OIB-derived magma. Group 2 shows higher Al2O3 but lower P2O5 contents and La/Yb ratios (<20) than Group 1, originating from asthenospheric mantle with input of delaminated lithospheric component. In combination with available data, it is proposed for the petrogenetic model of the Early Cretaceous thickened lithospheric delamination in response to the asthenospheric upwelling along the western Qinling orogenic belt.PostprintPeer reviewe

Expansion of the Tibetan Plateau during the Neogene

The appearance of detritus shed from mountain ranges along the northern margin of the Tibetan Plateau heralds the Cenozoic development of high topography. Current estimates of the age of the basal conglomerate in the Qaidam basin place this event in Paleocene-Eocene. Here we present new magnetostratigraphy and mammalian biostratigraphy that refine the onset of basin fill to ∼25.5 Myr and reveal that sediment accumulated continuously until ∼4.8 Myr. Sediment provenance implies a sustained source in the East Kunlun Shan throughout this time period. However, the appearance of detritus from the Qilian Shan at ∼12 Myr suggests emergence of topography north of the Qaidam occurred during the late Miocene. Our results imply that deformation and mountain building significantly post-date Indo-Asian collision and challenge the suggestion that the extent of the plateau has remained constant through time. Rather, our results require expansion of high topography during the past 25 Myr

Continental igneous rock composition: A major control of past global chemical weathering

The composition of igneous rocks in the continental crust has changed throughout Earth’s history. However, the impact of these compositional variations on chemical weathering, and by extension on seawater and atmosphere evolution, is largely unknown. We use the strontium isotope ratio in seawater [(87Sr/86Sr)seawater] as a proxy for chemical weathering, and we test the sensitivity of (87Sr/86Sr)seawater variations to the strontium isotopic composition (87Sr/86Sr) in igneous rocks generated through time. We demonstrate that the 87Sr/86Sr ratio in igneous rocks is correlated to the epsilon hafnium (εHf) of their hosted zircon grains, and we use the detrital zircon record to reconstruct the evolution of the 87Sr/86Sr ratio in zircon-bearing igneous rocks. The reconstructed 87Sr/86Sr variations in igneous rocks are strongly correlated with the (87Sr/86Sr)seawater variations over the last 1000 million years, suggesting a direct control of the isotopic composition of silicic magmatism on (87Sr/86Sr)seawater variations. The correlation decreases during several time periods, likely reflecting changes in the chemical weathering rate associated with paleogeographic, climatic, or tectonic events. We argue that for most of the last 1000 million years, the (87Sr/86Sr)seawater variations are responding to changes in the isotopic composition of silicic magmatism rather than to changes in the global chemical weathering rate. We conclude that the (87Sr/86Sr)seawater variations are of limited utility to reconstruct changes in the global chemical weathering rate in deep times