文档共享_Beijing SHRIMP Center

你好,请

In situ determination of U–Pb ages and Sr–Nd–Hf isotopic constraints on the petrogenesis of the Phalaborwa carbonatite Complex, South Africa [查看] Fu-YuanWu、Yue-HengYang、Qiu-LiLi、RogerH.Mitchell、J.BarryDawson、GüntherBrandl、MasakiYuhara

The Phalaborwa carbonatite Complex, situated in the northeastern part of South Africa, is characterized by copper and zirconium mineralization, and is composed principally of pyroxenites, phoscorite and carbonatite(banded and transgressive). The complex is transected by mafic dykes, and is geographically associated with a satellite syenite and minor granite intrusions. Zircon and baddeleyite U–Pb isotopic age determinations using CAMECA 1280 secondary ion mass spectrometry have shown that the outer pegmatitic pyroxenite at the Loolekop pipe was emplaced at 2060±4 Ma, and the main phoscorite at 2062±2 Ma. Both ages are identical to those of 2060±2 and 2060±1 Ma for the banded and transgressive carbonatites, respectively. The satellite syenite, which forms plug-like bodies outside of the border of the main complex, and the later mafic dyke have “similar” emplacement ages of 2068±17 and 2062±53 Ma, indicating that these intrusions were apparently near-synchronously emplaced. In contrast to other carbonatites, the Phalaborwa Complex is characterized by high initial Sr and low initial Nd and Hf isotopic compositions. In situ isotopic analyses of apatite,calcite, zircon and baddeleyite indicate that the primary magma was derived from an enriched mantle.As the complex was emplaced slightly earlier at ~2060 Ma than the nearby mafic phase of the Bushveld Complex (~2055 Ma), it is proposed that the Phalaborwa carbonatite magmatism was triggered by the same mantle plume activity, which partially melted the overlying lithospheric mantle. This contribution also highlights that isotopic studies used to constrain the genesis of ancient igneous complexes should concentrate on minerals with low parent/daughter elemental ratios, such as apatite and calcite for Sr isotopes, and zircon and baddeleyite for Hf isotopes.

U–Pb ages and Hf–O isotopes of zircons from Late Paleozoic mafic–ultramafic units in the southern Central Asian Orogenic Belt Tectonic implications and evidence for an Early-Permian mantle plume [查看] Ben-XunSu、Ke-ZhangQin、PatrickAsamoahSakyi、Xian-HuaLi、Yue-HengYang、HeSun、Dong-MeiTang、Ping-PingLiu、Qing-HuaXiao、SanjeewaP.K.Malaviarachchi

U–Pb age and Hf–O isotope data of zircons fromthe mafic–ultramafic complexes and their related rhyolites and granites in the Eastern Tianshan and Beishan regions were obtained by SIMS and LA-ICPMS. The Hf–O isotopic composition and Hf model age of the zircons are closely related to their tectonic setting and formation age. The zircons from the mafic–ultramafic complexes in the Bogeda–Haerlike and Jueluotage belts have higher εHf(t)(+8–+17) and lower δ18O (+4‰–+6‰), whereas most of the zircons in the gabbros, granites and rhyolite from theMiddle TianshanMassif and Beishan Rift display lower εHf(t) (0–+8) and higher δ18O (+5‰–+8‰).The positive εHf(t) and relatively lower δ18O values suggest that thesemafic–ultramafic complexes were derived from depleted mantle which was subjected to subduction-related modification processes. The Hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the Central Asian Orogenic Belt during the Paleozoic. The Hf model ages of the analyzed zircons, together with the regional geology suggest that the Beishan area had a northward subduction, possibly fromca. 900 Ma to ca. 400 Ma,whereas the Eastern Tianshan had a south-directed subductionmost likely fromca.600 Ma to ca. 310 Ma. Additionally, zircons with ca. 280 Ma U–Pb ages display wider and more scattered εHf(t) and δ18O variations than the relatively older and younger ones, which further support the Early Permian mantle plume model.

Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle [查看] Qiu-LiLi、Fu-YuanWu、Xian-HuaLi、Zhi-LiQiu、YuLiu、Yue-HengYang、Guo-QiangTang

Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts,which are important for tracking the evolution of subcontinental lithosphericmantle(SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and postemplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, singlegrain phlogopite Rb–Sr dating yields an isochron age of 485±4 Ma, U–Th–Pb analyses on perovskite give a 238U–206Pb age of 480.6±2.9 Ma and a 232Th–208Pb age of 478.9±3.9 Ma, and baddeleyite yields a 207Pb–206Pb age of 480.4±3.9 Ma. For Fuxian kimberlite, baddeleyite gives a 207Pb–206Pb age of 479.6±3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at ~480Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts fromthe SCLM, and can be used to constrain Hf isotope compositions (εHf(t)~−6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic–Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at ~1.3 Ga.Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred.

Age, geochemistry and tectonic setting of the Neoproterozoic (ca 830 Ma) gabbros on the southern margin of the North China Craton [查看] Xiao-LeiWan、Shao-YongJiang、Bao-ZhangDai、W.L.Griffin、Meng-NingDai、Yue-HengYang

Many studies have addressed the Paleozoic to Triassic collisional processes along the southern margins of the North China Craton (NCC), but little is known about the evolution of this margin during Precambrian time. This paper examines the geochronology and geochemistry of gabbros from the Luanchuan Group,which is located on the southern margin of the NCC. These gabbros are dated at ca 830 Ma by SHRIMP and LA–ICP-MS zircon U–Pb methods. They are characterized by high contents of TiO2 (2.21–4.45 wt%) and light-REEs (rare earth elements) ((La/Yb)N = 9.54–7.71). The gabbros have overall OIB (ocean island basalts)-like trace element patterns, without the positive Pb and Sr anomalies and negative Nb–Ta anomalies. In addition, they have low La/Nb (<1.5) and La/Ta (<30) ratios, indicating an origin in the asthenospheric mantle. The depletions of HFSEs (high field strength elements) (e.g. Zr–Hf and Ti) probably suggest that their source has been metasomatized by carbonates. The gabbros show negative εNd(t)(−1.5 to −3.0), suggesting crustal contamination or mixing with metasomatized lithospheric mantle. However, the low Th and U contents, constant incompatible-element ratios (e.g. Zr/Nb, La/Nb, La/Sm) and the absences of correlations of εNd(t) with MgO, Nb/Nb*, SiO2 or 1/Nd preclude significant crustal contamination. Alternatively, the wide range of Hf–isotope ratios in zircons (εHf(t) from −3.6 to +6.3) may imply the interaction between asthenosphere-derived melts and the metasomatized lithospheric mantle. Petrological modeling suggests that the gabbros may have been generated from the low-degree(2–3%) partial melting of lherzolite with 2% garnet at depths greater than 85 km. The gabbros may have been generated in a within-plate rift setting. The gabbros may best be correlated with the Neoproterozoic magmatic rocks in the NQB (North Qinling Belt), representing the relicts onto the southern margin of the NCC following the Neoproterozoic rifting between the NCC and the NQB. Therefore, the NCC and NQB may have been connected at ca 830 Ma. Moreover, the occurrence of the gabbros implies that the present southern margin of the NCC may have been mixed with late Mesoproterozoic to Neoproterozoic crustal materials from the NQB. The new findings imply that the southern margin of the NCC has been the locus of at least three extension-convergence cycles, and the reactivation of such tectonic margins may be more common in the geological record than previously recognized.

Amalgamation between the Yangtze and Cathaysia Blocks in South ChinaConstraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks [查看] Xian-HuaLi、Wu-XianLi、Zheng-XiangLi、Ching-HuaLo、JianWange、Mei-FangYe、Yue-HengYang

South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849±7Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926±15Ma and 891±12 Ma, respectively. All the studied rocks are characterized by highly positive Nd(T) (5.4–8.7) and Hf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, whichwere previously dated at ca. 970 Ma, are high in Al2O3 (15–20%) but low in MgO (<8%), and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, broadly similar to high-Al basaltic rocks in many volcanic arcs. The Beiwu andesitic to rhyolitic rocks have higherMgOthan the experimental melts of basaltic rocks, and their Al2O3 content decreases with increasing SiO2, similar to the regional coeval tonalites and granodiorites,suggesting their formation by crystal fractionation of basaltic parent magma. The Zhangcun volcanic rocks are high in SiO2 (mostly >69%), low in MgO (0.35–1.2%), and have nearly constant Al2O3 contents of 14–15%and relatively uniform trace element concentrations. Theywere generatedby remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890Maactive continental margin. These results and the 849±7Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890Ma and ca. 850 Ma.Previously reported 1.04–0.94Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking”at the eastern segment of the Sibao orogen, was completed at ca. 880Ma or soon after.

Zircon SHRIMP U-Pb and in-situ Lu–Hf isotope analyses of a tuff from Western Beijing Evidence for missing Late Paleozoic arc volcano eruptions at the northern margin of the North China block [查看] Shuan-HongZhang、YueZhao、BiaoSong、Yue-HengYang

Zircon SHRIMP U–Pb and in-situ Lu–Hf isotopic analyses via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of a tuff within the Upper Paleozoic from Western Beijing were carried out to give new constraints on volcano eruption ages and source area of the tuffs within the North China block (NCB). SHRIMP U–Pb zircon dating of the tuff yielded a 206Pb/238U weighted mean age of 296±4 Ma (95% confidence, MSWD=3.3), which is very similar to the emplacement age of the newly discovered Carboniferous calc-alkaline,I-type continental arc granitoid plutons in the Inner Mongolia Paleo-uplift (IMPU) on the northern margin of the NCB. In-situ Lu–Hf analysis results of most zircons from the tuff yielded initial 176Hf/177Hf ratios from 0.282142 to 0.282284 and εHf(t) values from −15.9 to −10.7. These Lu–Hf isotopic compositions are very similar to those of the Late Carboniferous granitoids in the IMPU, but are very different to those of the Central Asian Orogenic Belt (CAOB). Together with the sedimentary and tectonic analyses results, we inferred that the source area of the tuffs within the NCB is the IMPU instead of the CAOB. Therefore, some arc volcanoes once existed in the IMPU on northern margin of the NCB during the Late Carboniferous, but they were entirely eroded due to strong exhumation and erosion of the IMPU during the Late Carboniferous to Early Jurassic.

© BJSHRIMP 2013 - bjshrimp.cn