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Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai,NW China Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu

Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai,and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479 Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447 Ma to 368 Ma with a climax at ca. 400 Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280 Ma, respectively. Positive eHf(t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421 Ma old and also give positive eHf(t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400 Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period.Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400 Ma and the above mentioned change of Hf isotopic composition in the magma source.

Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu

The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U–Pb and Hf isotopic compositions and whole-rock geochemical and Nd–Sr isotopic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO+Na2O+K2O)=1.01–1.46 molecular ratios), with 0.6–5.6 wt.% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.38–0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393 Ma, consistent with a period of intensive magmatic activities and high temperaturemetamorphismin the Chinese Altai.While in situ zircon Hf isotopic analyses for these granites give predominantly positive εHf(t) values (+0.8 to+12.8), a few inherited zircons yield negative εHf(t) values from −12.5 to −1.53. The U–Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative εNd(t) values(−3.3 to−0.5) and relatively high initial 87Sr/86Sr ratios (0.7079–0.7266), distinct fromthose of the I-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5 kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672–861 °C. The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.

Geochronological and Geochemical study of Palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from NW Fujian, SE China Implications for the crustal evolution of the Cathaysia Block [查看] Long-MingLi、MinSun、YuejunWang、GuangfuXing、GuochunZhao、KedaCai、YuzhiZhang

U–Pb zircon dating of gneissic granite samples in northwestern Fujian Province, Southeast China, gave Neoarchean to Palaeoproterozoic ages (2.6–1.9 Ga) for the rounded zircon cores and Palaeoproterozoic upper intercept ages (1851 ± 21 Ma, 1857 ± 29 Ma, respectively) for zoned rims that have oscillatory growth zoning and yield eHf(t) values between 8.8 and +3.7 and TDM model ages between 2.1 and 2.6 Ga. These gneissic granites are peraluminous (A/CNK = 0.94–1.28), characterized by high SiO2 (68–72 wt.%), Al2O3 (14–15 wt.%) and low CaO, MgO, FeOt, TiO2 and P2O5 contents. They also possess relatively low REE contents and moderate LREE/HREE fractionation and display pronounced negative anomalies in Nb, Ta, Sr, P and Ti. All these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks.Clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high MgO, Ni and Cr, but low contents of TiO2, and are characterized by high CaO/Al2O3 (4.1–5.3),low Al2O3/TiO2 ratios (8.7–9.5) and HREE depletion. These features are similar to the typical Al-depleted type komatiites. Their geochemical characteristics, such as high (Gd/Yb)N and low HREE, Y, Zr and Hf suggest partial melting of the upper mantle source with residual garnet. The negative Nb, Ta and Ti anomalies indicate a strong arc-related affinity.Our results and recently published data for granitic and mafic basement rocks in this region, reveal that 1.85 Ga was an important period for the evolution of the Cathaysia Block, possibly related to the Columbia supercontinent assembly. Large amount of granitic magma was emplaced, probably as a result of partial melting of old Neoarchean to Palaeoproterozoic materials.

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