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SHRIMP U-Pb zircon geochronology of high-grade rocks and charnockites from the eastern Amery Ice Shelf and southwestern Prydz Bay, East Antarctica Constraints on Late Mesoproterozoic to Cambrian tectonot [查看] XiaochunLiu、YueZhao、BiaoSong、JianLiu、JianjunCui

The eastern Amery Ice Shelf (EAIS) and southwestern Prydz Bay are situated near the junction between the Late Neoproterozoic/Cambrian high-grade complex of the Prydz Belt and the Early Neoproterozoic Rayner Complex. The area contains an important geological section for understanding the tectonic evolution of East Antarctica. SHRIMP U–Pb analyses on zircons of felsic orthogneisses and mafic granulites from the area indicate that their protoliths were emplaced during four episodes of ca. 1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma and ca. 1060–1020 Ma. Subsequently, these rocks experienced two episodes of high-grade metamorphism at N970 Ma and ca. 930–900 Ma, and furthermore, most of them (except for some from the Munro Kerr Mountains and Reinbolt Hills) were subjected to high-grade metamorphic recrystallization at ca.535 Ma. Two suites of charnockite, i.e. the Reinbolt and Jennings charnockites, intrude the Late Mesoproterozoic/Early Neoproterozoic and Late Neoproterozoic/Cambrian high-grade complexes at N955 Ma and 500 Ma, respectively. These, together with associated granites of similar ages, reflect late- to post-orogenic magmatism occurring during the two major orogenic events. The similarity in age patterns suggests that the EAIS–Prydz Bay region may have suffered from the same high-grade tectonothermal evolution with the Rayner Complex and the Eastern Ghats of India. Three segments might constitute a previously unified Late Mesoproterozoic/Early Neoproterozoic orogen that resulted from the long-term magmatic accretion from ca. 1380 to 1020 Ma and eventual collision before ca. 900 Ma between India and the western portion of East Antarctica. The Prydz Belt may have developed on the eastern margin of the Indo-Antarctica continental block, and the Late Neoproterozoic/Cambrian suture assembling Indo-Antarctica and Australo-Antarctica continental blocks should be located southeastwards of the EAIS–Prydz Bay region.

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.

The 1.75–1.68 Ga anorthosite-mangerite-alkali granitoid-rapakivi granite suite from the northern North China Craton Magmatism related to a Paleoproterozoic orogen [查看] Shuan-HongZhang、Shu-WenLiu、YueZhaoa、Jin-HuiYang、BiaoSong、Xiao-MingLiu

The AMGRS is a suite of anorthosite-mangerite-alkali granitoid-rapakivi granite in the northern North China Craton (NCC).New zircon SHRIMP and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) U–Pb dating of the Damiao anorthosite, Changsaoying K-feldspar granite, Lanying anorthosite and quartz syenite and the Gubeikou K-feldspar granite yielded emplacement ages of 1726±9 Ma, 1753±23 Ma, 1739±43 Ma, 1712±15 Ma and 1692±19 Ma, respectively. Combined with previous geochronological data from the Damiao norite and mangerite and Shachang rapakivi granite, it indicates that the whole AMGRS was intruded between 1750 Ma and 1680 Ma. The Damiao anorthosites and norites exhibit a high Mg#, and this decreases from mafic (up to 63) to felsic (<10) end members. The alkali granitoids of the AMGRS are characterized by high contents of alkali (K2O+Na2O), HSFE, TFeO/MgO, molecular (K2O+Na2O)/Al2O3 and Ga/Al values, that are typical features of A-type granites. All of the diverse components of the AMGRS have similar Sr–Nd–Hf isotopic compositions with low whole-rock initial 87Sr/86Sr ratios of 0.703–0.704, εNd(T) values of −5.0 to −6.3 (with TDM model ages of 2.34 to 2.58 Ga), and zircon εHf(T) values ranging from −4.1 to −7.5 (with Hf isotopic model TDM and TCDM ages ranging from 2.32 Ga to 2.43 Ga and 2.70 Ga to 2.88 Ga,respectively).This suggests that these rocks are the fractional crystallization products of one parental magma. Zircon Hf and wholerock Sr–Nd–Pb isotopic systematics combined with high La/Yb (>10) and intermediate Th/Ta ratios, suggest that the parental magma of the AMGRS could have been derived from EM-I lithosphere mantle enriched by recycled Archean continent crustal materials, accompanied by some assimilation of lower crustal components. The mafic to felsic compositions of the AMGRS are different from typical bimodal rocks formed in an anorogenic intra-continent rift setting, but resemble those emplaced during postcollisional/post-orogenic extensional tectonic setting. This is compatible with their age of <1.85 Ga, indicating they postdate the continent–continent collision between the Western and Eastern blocks of the NCC at ∼1.85 Ga.

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