oping Xia

PALEOPROTEROZOIC CRUSTAL GROWTH IN THE WESTERN BLOCK OF THE NORTH CHINA CRATON: EVIDENCE FROM DETRITAL ZIRCON Hf AND WHOLE ROCK Sr-Nd ISOTOPIC COMPOSITIONS OF THE KHONDALITES FROM THE JINING COMPLEX

Detrital zircons from the khondalites in the Western Block of the North China Craton were mainly derived from a 1.9 to 2.1 Ga Paleoproterozoic provenance. Most of the more than 200 detrital zircons analyzed in this study have positive initial εHf values and a wide range of Hf depleted mantle model ages (TDMC) that range from 2.08 Ga to 2.74 Ga, with a peak at 2.24 to 2.40 Ga. The whole rock Nd isotopic analyses for the khondalite samples give quite uniform 143Nd/144Nd ratios from 0.511188 to 0.511858, corresponding to initial εNd (2.10Ga) values ranging from -1.6 to +0.6. In contrast, Sr isotopic compositions of these samples are more varied, with initial 87Sr/86Sr ratios at 2.10 Ga ranging from 0.7017 to 0.7076. In the εNd versus 87Sr/86Sr correlation diagram, most samples plot near the mixing hyperbola of the depeleted mantle and crust, with the contribution of depleted mantle about 65 to 75 percent. These results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the Western Block at 2.1 Ga, a period of magmatic quiescence in many cratons of the world.

U–Pb and Hf isotopic study of detrital zircons from the Lüliang khondalite, North China Craton, and their tectonic implications

Two types of metasedimentary rocks occur in the Trans-North China Orogen of the North China Craton. One type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale, called khondalites, as found in the Luliang Complex; rocks of the other type are also highly metamorphosed but less mature, as represented by the Wanzi supracrustal assemblage in the Fuping Complex. U–Pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1.9–2.1 Ga Palaeoproterozoic rocks. These detrital zircons display a wide range of e Hf values from −16.0 to +9.2 and give Hf isotopic model ages mostly around 2.3 Ga. The high positive e Hf values approach those for the depleted mantle at 2.1 Ga, highlighting a juvenile crustal growth event in Palaeoproterozoic times. Hf isotopic data also imply that c. 2.6 Ga old crustal material was involved in the Palaeoproterozoic magmatic event. These data are similar to those for the khondalitic rocks from the interior of the Western Block of the North China Craton, suggesting a common provenance. In contrast, other metasedimentary rocks in the Trans-North China Orogen, such as the Wanzi supracrustal assemblage in the Fuping Complex, have a source region with both Palaeoproterozoic and Archaean rocks. Their detrital zircon Hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile Palaeoproterozoic magmatic input. These rocks are similar to the coevally deposited meta-sedimentary rocks in the interior of the Eastern Block. We propose that the Luliang khondalites were deposited on the eastern margin of the Western Block in a passive continental margin environment and were thrust eastward later during collision with the Eastern Block. Other metasedimentary rocks in the Trans-North China Orogen were deposited on the western margin of the Eastern Block in a continental arc environment. Our data support the eastward subduction model for the Palaeoproterozoic tectonic evolution of the North China Craton.

Quasi-simultaneous determination of U-Pb and Hf isotope compositions of zircon by excimer laser-ablation multiple-collector ICPMS

A method for quasi-simultaneous determination of U-Pb ages and Hf isotope compositions on a single zircon spot is developed, by use of a multiple-collector ICPMS coupled with an excimer laser-ablation system. The ICPMS used in this study is equipped with variable zoom lens to change the dispersion of the analyzer and bring ion beams into the collector coincidentally. This feature allows rapid switching between U-Pb and Hf collector configuration, which is used to quasi-simultaneously determine the zircon U-Pb and Hf isotope compositions. The analytical results on three reference zircon standards, 91500, CZ3 and Plesovice and one nature zircon sample, agree with literature/known values, thereby demonstrating feasibility of the method.

THE 390 Ma HIGH-T METAMORPHIC EVENT IN THE CHINESE ALTAI: A CONSEQUENCE OF RIDGE-SUBDUCTION?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

LA-ICP-MS U-Pb Zircon Geochronology of the Yushulazi Group in the Eastern Block, North China Craton

The Paleoproterozoic Jiao-Liao-Ji belt in the North China craton is composed mainly of the Liaohe Group (and its equivalents) and the Liaoji Granites. They are uncomformably overlain by the sedimentary Yushulazi Group, which itself is overlain by the Neoproterozoic (0.9-0.8 Ga) Yongning Group. The Yushulazi Group has long been considered as a Proterozoic assemblage, but its depositional age has never been precisely constrained in terms of modern geochronology. Cathodoluminescence images reveal that most detrital zircon grains from this formation are characterized by concentric-oscillatory zoning, with comparatively low luminescence and high Th/U ratios, suggesting an igneous origin, whereas minor grains have structureless overgrowth or recrystallization rims that are characterized by low Th/U ratios, indicating a metamorphic origin. The results of LA-ICP-MS U-Pb zircon analyses show that the detrital zircon grains have concordant U-Pb ages of two populations—i.e., 1.05-1.37 Ga and 1.87-2.51 Ga—with an inherited zircon core giving a concordant age of 3.35 Ga. The U-Pb age range of 1.87-2.51 Ga is in good accordance with that of the Paleoproterozoic Liaohe Group and other basement rocks in the Eastern Block of the North China craton. Zircon ages of 1.05-1.31 Ga are, for the first time, reported for the Precambrian basement. The recognition of these Mesoproterozoic (1.05-1.31 Ga) igneous zircons from the Yushulazi Group indicate the existence of Grenvillian-aged magmatism in the Eastern Block of the North China craton. The Yushulazi Group must have deposited at some time between 1.05 Ga and 0.9 Ga, prior to sedimentation of the 0.9-0.8 Ga Yongning Group and it is not the upper part of the Paleoproterozoic Liaohe Group, as previously suggested.

An Improved Fe—Ni Sulfide Fire Assay Method for Determination of Re, Platinum Group Elements, and Os Isotopic Ratios by Inductively Coupled Plasma- and Negative Thermal Ionization—Mass Spectrometry

An improved Fe–Ni sulfide fire assay method has been developed for determination of Re, the platinum group elements (PGE), and Os isotopic ratios using inductively coupled plasma–mass spectrometry (ICP-MS) and negative thermal ionization–mass spectrometry (NTI-MS). Recovery of Re using the neoclassical NiS fire assay technique is very low, but recoveries of up to 75% can be achieved by using Fe–Ni sulfide as a collector and Na2B4O7 as a flux. Using isotope dilution for determination of Re, a number of standard reference materials were analyzed for PGE and Re, and the results are consistent with their certified values. Multiple analyses of the reference standard GBW 07290 produced precisions ranging from 2.2% for Os to 5.9% for Ir. The detection limits are 2 pg g−1 for Ru, 1.5 pg g−1 for Rh, 25 pg g−1 for Pd, 23 pg g−1 for Re, 0.7 pg g−1 for Os, 1 pg g−1 for Ir, and 6 pg g−1 for Pt. The low procedural blank of Os (1.3 pg g−1) makes the Fe–Ni sulfide fire assay suitable for analysis of Os isotopic compositions. Using NTI-MS, the 187Os/188Os ratios were measured for WPR-1 and they are consistent with literature values.

Carboniferous and Permian evolutionary records for the Paleo‐Tethys Ocean constrained by newly discovered Xiangtaohu ophiolites from central Qiangtang, central Tibet

Reconstructing the evolutionary history of the Paleo-Tethys Ocean remains at the center of debates over the linkage between Gondwana dispersion and Asian accretion. Identifying the remnants of oceanic lithosphere (ophiolites) has very important implications for identifying suture zones, unveiling the evolutionary history of fossil oceans, and reconstructing the amalgamation history between different blocks. Here we report newly documented ophiolite suites from the Longmu Co-Shuanghu Suture zone (LSSZ) in the Xiangtaohu area, central Qiangtang block, Tibet. Detailed geological investigations and zircon U-Pb dating reveal that the Xiangtaohu ophiolites are composed of a suite of Permian (281–275 Ma) ophiolites with a nearly complete Penrose sequence and a suite of Early Carboniferous (circa 350 Ma) ophiolite remnants containing only part of the lower oceanic crust. Geochemical and Sr-Nd-O isotopic data show that the Permian and Carboniferous ophiolites in this study were derived from an N-mid-ocean ridge basalts-like mantle source with varied suprasubduction-zone (SSZ) signatures and were characterized by crystallization sequences from wet magmas, suggesting typical SSZ-affinity ophiolites. Permian and Carboniferous SSZ ophiolites in the central Qiangtang provide robust evidence for the existence and evolution of an ancient ocean basin. Combining with previous studies on high-pressure metamorphic rocks and pelagic radiolarian cherts, and with tectonostratigraphic and paleontological data, we support the LSSZ as representing the main suture of the Paleo-Tethys Ocean which probably existed and evolved from Devonian to Triassic. The opening and demise of the Paleo-Tethys Ocean dominated the formation of the major framework for the East and/or Southeast Asia.

Zircon Hf isotope of Yingfeng Rapakivi granites from the Quanji Massif and ∼2.7 Ga crustal growth

The Quanji (全吉) Massif is located in the Northwest China, which is interpreted as a micro-continent that is composed of metamorphic basement and stable cover strata. There are some controversies of genetic relationship between the Quanji Massif and the major cratons in China. In this study, we obtained in situ zircon U-Pb and Hf isotopic compositions of the Yingfeng (鹰峰) rapakivi granites from the northwest Quanji Massif by application of LA-MC-ICP-MS technique. Twenty U-Pb age measurements points are concordant or near concordant, and their weighted mean 207Pb/206Pb age is 1 793.9±6.4 Ma (MSWD= 1.09), yields an upper intercept age of 1 800±17 Ma (MSWD=0.41); 19 Hf isotope measurements yield a two-stage Hf model ages (TDM2) of 2.63 to 2.81 Ga, with a weighted average age of about 2.70±0.02 Ga and ɛHf(t) values variate between -8.91 to -5.35. This indicates that magma source of the Yingfeng rapakivi granites were produced from partial melting of late stage of Neoarchean juvenile crust, and suggests a significant crustal growth event occurred in the Quanji Massif at that time. The Quanji Massif might be an ancient continental segment detached from the Tarim Craton based on the crustal growth history and other geological records. The Tarim Craton (including the Quanji Massif) and the North China Craton had a similar or homological early crustal evolution around ∼2.7 Ga, which implies that Tarim Craton might be one of the component parts of North China Craton.

Low‐δ18O Rhyolites From the Malani Igneous Suite: A Positive Test for South China and NW India Linkage in Rodinia

The Malani Igneous Suite (MIS) in NW India represents one of the best preserved silicic large igneous provinces. Voluminous silicic lavas of the MIS erupted between ca. 780-750 Ma. Zircon grains from rhyolite and dacite lavas have oxygen isotopic compositions that include depleted (δ18O = 4.12 to -1.11‰) and enriched (δ18O = 8.23-5.12‰) signatures. The low-δ18O zircon grains have highly radiogenic Hf isotopic compositions (ƐHf(t)= +13.0 to +3.6), suggesting high temperature bulk cannibalization of upper level juvenile mafic crust as an essential mechanism to produce the low-δ18O felsic magma. Xenocrystic zircon grains in dacites have high δ18O and low ƐHf(t) values for magmas older than 800 Ma, reflecting a dramatic transition in tectono-thermal regime in NW India during 800-780 Ma. A synchronous transition also occurred in South China and Madagascar, suggesting a spatially linked geodynamic system. NW India and South China together with Madagascar and the Seychelles lay either along the periphery of Rodinia or outboards of the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.

Petrogenesis of the Permian Intermediate-Mafic Dikes in the Chinese Altai, Northwest China: Implication for a Postaccretion Extensional Scenario

The Central Asian Orogenic Belt is a long-lived accretionary orogen, and the late Paleozoic has been considered to be a critical period for the terminal amalgamation of its three tectonic collage systems. However, the exact timing of amalgamation and the geological process of such a huge accretionary orogenic belt are poorly understood. This study presents new geochronological and geochemical data for Permian intermediate-mafic dikes in the Chinese Altai, a key region between the Mongolian and the Kazakhstan collage systems. According to mineral assemblages and petrographic textures, the intermediate-mafic dikes can be categorized as gabbronorite and quartz diorite. The gabbronoritic and quartz dioritic dikes have zircon U-Pb ages of 276.7 ± 2.9 and 273.2 ± 4.3 Ma, respectively. The gabbronorites are characterized by low SiO2 (47.1–51.3 wt%) and high MgO (5.33–8.46 wt%), together with medium Cr (71.2–95.7 ppm) and Ni (80.6–192 ppm) contents. Geochemical modeling indicates that the parental magma was possibly contaminated by 4%–12% crustal materials. Zircon εHf(t) (+13.2 to +16.7) and whole-rock εNd(t) (+4.9 to +6.1) values as well as moderate Sm/Yb ratios (1.75–1.89) imply that the parental magma might have originated from a depleted mantle source dominated by spine lherzolite. In contrast, the quartz diorites exhibit higher SiO2 (57.3–58.3 wt%) and lower whole-rock εNd(t) (∼+2.5) and zircon εHf(t) (+9.1 to +14.4) values, implying that they have a magma source unlike the depleted mantle of the gabbronorites. The parental magma may be derived from mafic lower crust. The quartz diorites have high Y (>39.8 ppm) and heavy rare earth element (e.g., Yb >3.64 ppm) concentrations as well as low Sr/Y (<12) ratios, consistent with geochemical fingerprints of a magma reservoir at shallow depths (<10 kb). Major element compositions of the quartz diorites are comparable to those of intermediate liquids generated by ∼40% partial melting of alkali-enriched basaltic rocks at conditions of T = 1050°–1100°C and P = 8 kbar. Such a high geothermal gradient is inferred to be a consequence of intraplating and/or underplating of hot basaltic magmas in an extensional setting, which may shed light on the ubiquitous tectonic scenario after amalgamation of tectonic collages.