Refertilization of serpentinites in the Sulu UHP terrane,Eastern China: constraints from geochronology,mineral composition,geochemistry, and Re–Os isotopes

ABSTRACT

Serpentinites from Junan (JN), Rizhao (RZ), and Rongcheng (RC) in the Sulu ultra-high-pressure (UHP) terrane, China, were analysed for U–Pb zircon geochronology, mineral chemistry, whole-rock major and trace element chemistry (including rare-earth elements (REEs) and platinum-group elements (PGEs)), and Re–Os isotopes, in order to better constrain their petrogenesis and geodynamic process. The serpentinite zircons yield two age groups: 731 ± 10 to 780 ± 10 Ma for relic magmatic zircon cores, which may indicate early crystallization and emplacement of the peridotite in the Yangtze crust, and 209 ± 2 to 218 ± 3 Ma for metamorphic zircon, which coincides with Triassic UHP metamorphism. The spinels in the serpentinites exhibit significant Cr# variation (0.6–0.91) and have undergone multi-stage metamorphism. The serpentinites are characterized by enrichment in incompatible trace elements, low Ni and IPGE concentrations, and high Pd/Ir ratios, and the bulk-rock major elements plot in the ultramafic cumulate region. Their Re and Os concentrations are similar to those of typical orogenic peridotite, but they have high ¹⁸⁷Os/¹⁸⁸Os ratios (0.12433–0.14423). We believe that the serpentinite’s protolith consisted of cumulates from an asthenosphere-derived melt that intruded into the continental crust of the Yangtze craton in the Neoproterozoic. These cumulates were later subducted and metamorphosed during the subduction of the Yangtze craton in the Triassic. The serpentinites underwent melt–rock interactions and fluid enrichment, both prior to and during serpentinization.     

Mineralization,geochronology, and Pb isotope studies of the shoshonitic lava-hosted Nariniya Pb deposit,central Tibet: linking ore formation to post-collisional potassic magmatism

Abstract

A newly discovered, shoshonitic lava-hosted Pb deposit at Nariniya in central Tibet provides an excellent example to help improve our understanding of the linkage between post-collisional potassic magmatism and ore formation in Tibet. The Pb ores exist as veins or veinlets in NWW-striking fracture zones within the potassic lava (trachyte). The veins contain quartz, galena, pyrite, and sericite (muscovite) as well as minor chalcopyrite, sphalerite, calcite, and dolomite with sericitization, pyritization, and minor silicification. The ⁴⁰Ar–³⁹Ar plateau age of the hydrothermal muscovite is 37.95 ± 0.30 Ma, which represents the Pb mineralization age. This obtained age is indistinguishable, within analytical error, from the zircon U–Pb age of 37.88 ± 0.22 Ma for potassic lava. Therefore, the ore formation can be genetically linked to potassic magmatism. Galena has similar Pb isotopic composition to magmatic feldspar from the host lava, suggesting the derivation of Pb from the magmatic system. Previous studies have suggested that S- and ore-forming fluids are of magmatic origin. Published data show that the Nariniya volcanic rocks are acidic, shoshonitic, akakitic, peraluminous, and enriched in Sr–Nd–Pb isotopes. Thus, they are geochemically different from other potassic volcanic rocks (no adakitic affinity) in the North Qiangtang terrane, but similar to the 46–38 Ma high-K calc-alkaline peraluminous adakitic rocks in this terrane and the late Eocene Cu-generating potassic porphyries from the Sanjiang region of eastern Tibet. As such, the Nariniya potassic magma likely originated from melting of subducted continental crust, with or without interaction with the overlying enriched mantle. Such post-collisional potassic rocks in Tibet are thought to be potential targets for prospecting of both Pb–Zn and porphyry Cu ores. Note that other ore styles (in addition to the Nariniya ore style) may exist in the potassic volcanic districts of Tibet.     

The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust

ABSTRACT

The nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite components are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)-like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab-derived components into the overlying mantle wedge causes formation of diabase dikes with supra-subduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange.     

Detrital zircon ages and Hf isotopic compositions of metasedimentary rocks in the Wuqia area of Southwest Tianshan,NW China: implications for the early Paleozoic tectonic evolution of the Tianshan orogenic belt

ABSTRACT

The Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of ?22.0 to +11.3 and two-stage model ages (TDM₂) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (?15.0 to ?0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.     

利用包裹体中气体地球化学特征与源岩生气模拟实验探讨鄂尔多斯盆地靖边气田天然气来源

鄂尔多斯盆地上、下古生界地层包裹体气体与气藏中气体地球化学性质对比表明:上古生界气藏中气体与包裹体中气体地球化学性质相似,气藏中气体的地球化学性质能代表成藏初期气体的原始特征;而下古生界气藏中气体与包裹体中气体的地球化学性质差别很大,下古生界气藏中的气体与下古生界源岩模拟生成气体也有非常大的差别。因此,下古生界气藏中的气体不能代表来自下古生界源岩产生天然气。结合前人关于奥陶系源岩的模拟生成天然气、包裹体中气体以及靖边气田天然气的地球化学特征,提出来源于奥陶系的天然气应具有δ¹³C₁<-38‰、δ¹³C₂<-28‰的特征。下古生界地层包裹体中气体与气藏中气体地球化学性质对比表明,下古生界气田天然气乙烷碳同位素的变化范围也比甲烷碳同位素的变化范围大很多,乙烷碳同位素不适合作为判断靖边气田天然气来源的标准。在此基础上,以上古生界天然气甲烷碳同位素的平均值(-32.90‰)与下古生界δ¹³C₁<-38‰天然气甲烷碳同位素的平均值(-39.04‰)分别作为上、下古生界来源天然气甲烷碳同位素的界限值,通过简单计算认为靖边气田大约85%的天然气来源于上古生界煤系。     

西天山CuNi-VTiFe复合型矿化镁铁-超镁铁杂岩 ——哈拉达拉岩体成岩成矿背景特殊性讨论

西天山哈拉达拉镁铁-超镁铁杂岩为一CuNi-VTiFe复合型矿化岩体,主要由橄长岩、橄榄辉长岩、角闪辉长岩、辉长岩和辉绿岩组成。获得岩体中橄榄辉长岩SHRIMP锆石U-Pb年龄为308.8±1.9Ma,辉长岩SHRIMP锆石U-Pb年龄307.3±8.2Ma。岩石具堆晶结构、辉长-辉绿结构,岩体韵律层发育;各类岩石稀土和微量元素配分曲线模式相似,多具Eu,Sr正异常,它们可能为同一岩浆结晶分异演化的产物;Sr-Nd同位素特征(87Sr/86Sr初始比值=0.703913～0.705259,εNd(t)=4.00～8.42)表明原始岩浆来自于亏损地幔源区。推测哈拉达拉岩体形成于后碰撞造山早期伸展环境、叠加近同期地幔柱活动的特殊地质背景中。     

塔里木盆地西北缘奥陶系碳酸盐岩中两种来源热流体的地质与地球化学证据

塔里木盆地西北缘西克尔-巴楚-柯坪地区处于巴楚-柯坪断隆带活动区,构造活动强烈.该区出露的中下奥陶统鹰山组发育大量的热液蚀变现象,既有溶蚀形成的大小孔洞、疏松灰岩层,也有各类充填的脉体,如萤石矿、硫磺石膏矿等.通过野外露头观察和采样,并对受热流体作用的样品进行了包裹体测温测盐、微量元素和硫同位素分析.结果显示研究区主要有两种流体来源:岩浆热液和地层热液.研究表明:(1)岩浆热液主要影响研究区南西段,形成萤石、重晶石、方铅矿等矿物,流体包裹体盐度低(0.5％ ～7.5％ NaCleqv)、均一化温度相对较高(142 ～ 283℃);地层热液作用范围则主要位于研究区北东段,生成白云石、方解石、石英等矿物,流体包裹体盐度高(16％ ～ 23.3％ NaCleqv)、均一化温度低(70～ 138℃);而研究区中段同时受到两种来源热液的混合作用,包裹体盐度(7.5％～18.3％ NaCleqv)和均一化温度(142 ～ 163℃)介于前两者之间;(2)热液硬石膏、硫磺的硫同位素富含32S,δ4S平均为-9.57％,说明硬石膏是H2S气体与灰岩反应的产物,硫化氢很可能来自有机物的热裂解.     

Sulphur- and lead-isotope geochemistry of the Arapuçandere lead–zinc–copper deposit,Biga Peninsula,northwest Turkey

The Arapuçandere Pb–Zn–Cu ore body is a typical vein-type lead–zinc deposit of the Biga Peninsula, and is currently being mined for lead and zinc. In the study area, Permian–Triassic metamorphic rocks, Triassic metaclastic and metabasic rocks, Oligocene–Miocene granitoids, Miocene volcanic rocks, and Quaternary terrigenous sediments crop out. The ore deposits developed as Pb–Zn–Cu-bearing veins along faults in Triassic metasandstone and metadiabase. Microscopic studies reveal that the veins contain galena, sphalerite, chalcopyrite, pyrite, marcasite, covellite, and specular hematite as ore minerals, and quartz, calcite, and barite as gangue minerals. Analysed sulphur-isotope compositions (δ³⁴S_VCDT) of galena, sphalerite, and chalcopyrite range from ? 5.9 to ? 1.9‰ (average ? 3.4‰), from ? 5.5 to ? 1.7‰ (average ? 4.2 ‰), and from ? 3.5 to ? 0.9‰ (average ? 2.6‰), respectively; that of H₂S in the hydrothermal fluid was in the calculated range of ? 5.8 to +0.1‰ (average ? 2.5‰). These isotopic values suggest that magmatic sulphur dominates in sulphides, mixed with minor, isotopically light sulphur. Because no contemporaneous magmatic activity is associated with mineralization, it may be assumed that sulphur was leached from the surrounding Triassic units, mainly from metabasic, partly from metaclastic rocks. Lead-isotope studies indicate a model age of 114–63 Ma for the lead reservoir, in accord with possible sulphur-bearing local source rocks. Thus, the sulphur and lead deposited in the studied ore veins were probably leached from Triassic metabasic and metaclastic rocks some time during the Early Cretaceous to the Palaeocene.     

Geochronological evidence and tectonic significance of Carboniferous magmatism in the southwest Trabzon area,eastern Pontides,Turkey

The northern and southern zones of the eastern Pontides (northeast Turkey) contain numerous plutons of varying ages and compositions. Geochemical and isotopic results on two Hercynian granitoid bodies located in the northern zone of the eastern Pontides allow a proper reconstruction of their origin for the first time. The intrusive rocks comprise four distinct bodies, two of which we investigated in detail. Based on LA–ICP–MS U–Pb zircon dating, the Derinoba and Kayadibi granites have similar ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U Concordia ages of 311.1 ± 2.0 and 317.2 ± 3.5 million years for the former and 303.8 ± 1.5 million years for the latter. Aluminium saturation index values of both granites are between 0.95 and 1.35, indicating dominant peraluminous melt compositions. Both intrusions have high SiO₂ (74–77 wt.%) contents and show high-K calc-alkaline and I- to S-type characteristics. Primitive mantle-normalized element diagrams display enrichment in K, Rb, Th, and U, and depletion in Ba, Nb, Ta, Sr, P, and Ti. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies with La_cn/Yb_cn?=?4.6–9.7 and Eu_cn/Eu*?=?0.11–0.59 (Derinoba), and La_cn/Yb_cn?=?2.7–5.5 and Eu_cn/Eu*?=?0.31–0.37 (Kayadibi). These features imply crystal-melt fractionation of plagioclase and K-feldspar without significant involvement of garnet. The Derinoba samples have initial ?Nd values between –6.1 and –7.1 with Nd model ages and T _DM between 1.56 and 2.15 thousand million years. The Kayadibi samples show higher initial ?Nd_(I) values, –4.5 to –6.2, with Nd model ages between 1.50 and 1.72 thousand million years. This study demonstrates that the Sr isotope ratios generally display negative correlation with Nd isotopes; Sr isotope ratios were lowered in some samples by hydrothermal interaction or alteration. Isotopic and petrological data suggest that both granites were produced by the partial melting of early Palaeozoic lower crustal rocks, with minor contribution from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the eastern Pontides.