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    Geochemistry of TTG and TTG-like gneisses from Lushan-Taihua complex in the southern North China Craton Implications for late Archean crustal accretion. [查看] Xiao-LongHuang、YaolingNiu、Yi-GangXu、Qi-JunYang、Jun-WeiZhong
    The Late Archean Taihua complex, mainly composed of amphibolite and TTG and TTG-like gneisses, is volumetrically most important metamorphic rock suites scattered along the southern margin of the North China Craton (NCC). Zircon SHRIMP U–Pb dating shows two episodes (2765±13 and 2723±9 Ma) of Archean magmatism in the Lushan area with distinct geochemical features. The early (2765±13 Ma) suite (TTG-like gneisses) has low-SiO2 (52.5–66.1 wt%), high-Mg# (0.47–0.68), low HREE (YbN = 3.0–5.4) and Y (8.07–13.9 ppm) with low to moderate (La/Yb)N (6.7–37.1) and Sr/Y (25.9–119.3). The younger (2723±9 Ma) suite (TTG gneisses) has high-SiO2 (63.5–74.3 wt%), low-Mg# (0.13–0.52), very low REE (YbN < 1.8) and Y (<4 ppm) with a wide range of (La/Yb)N (5.2–86.6), Sr/Y (71.4–949) and showing Eu/Eu* > 1 (1.20–2.43). Both suites show pronounced negative Nb–Ta and Ti anomalies on the primitive mantle-normalized spidergram. The TTG-like gneiss suite has similar bulk-rock Nd and zircon Hf model ages (∼3.0 Ga) with εNd(t) > 0 (0.26–1.46), and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet + clinopyroxene + rutile±amphibole, which favors subducted slab model for the late Archean TTG. The TTG gneiss suite has abundant relic zircons (2.95–2.80 Ga) with εNd(t)<0 (−1.31 to −0.23), which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet-amphibolite residue (garnet + amphibole±rutile). Significant high-pressure fractional crystallization (garnet±amphibole) and accumulation (plagioclase) are also required in the petrogenesis. The Lushan amphibolite with nearly flat primitive mantle-normalized trace-element pattern is interpreted to represent early ocean crust metamorphism. These observations suggest a possible model of late Archean crustal accretion from ocean crust to continental terrain in the southern North China Craton.
    Age and duration of the Emeishan flood volcanism, SW China Geochemistry and SHRIMP zircon U– Pb dating of silicic ignimbrites, post-vo lcanic Xuanwei Formation and clay tuff at the Chaotian section [查看] BinHe、Yi-GangXu、Xiao-LongHuang、Zhen-YuLuo、Yu-RuoShi、Qi-JunYang、Song-YueYu
    The age and duration of the Emeishan basalts (SW China) remain poorly constrained largely due to the severe thermotectonic overprinting of the Ar–Ar system and failure to obtain zircon separates from erupted basalts. In an attempt to solve this problem, geochemical analyses and SHRIMP zircon U–Pb dating have been carried out on rare felsic ignimbrite in the uppermost of the Emeishan lava succession, the Xuanwei Formation which immediately overlies the Emeishan basalts and a clay tuff at the Middle–Late Permian boundary at the Chaotian section. Clastic rocks of the lowermost Xuanwei Formation (Group 1) in eastern Emeishan large igneous province (LIP) have a geochemical affinity to the Emeishan felsic volcanic rocks, whereas the overlying sediments (Group 2) are compositionally more akin to mafic components of the Emeishan traps. This is the reverse of volcanic sequence of the central Emeishan LIP where the felsic extrusives sit above predominant mafic rocks. It is likely that the clastic rocks are water-transported sediments resulting from erosion of the volcanic rocks in the center of the Emeishan LIP. This interpretation is further supported by the general agreement between the age of the lowermost Xuanwei Formation (257± 4 Ma; 260±5 Ma) and that of the silicic ignimbrite (263±4 Ma) and the clay tuff at the Middle–Late Permian boundary at Chaotian(260± 5 Ma). These ages, interpreted as the termination age of the Emeishan volcanism, are indistinguishable within error from the Middle–Late Permian boundary age (260.4± 0.4 Ma) and the main stage (259–262±3 Ma) of the Emeishan volcanism inferred from dating of mafic and alkaline intrusions in the Emeishan LIP. All these suggest that the emplacement of the Emeishan volcanism took place over a very short interval. Moreover, the temporal link and geochemical analyses suggest that the Chaotian clay at the Middle–Late Permian boundary was genetically related to the Emeishan silicic volcanism. This, together with the fact at that both the Emeishan basalts and the Chaotian clay rest on the Maokou Formation, leads us to infer that the Emeishan basalt was emplaced at the Middle–Late Permian boundary. In this sense, the Emeishan volcanism can be regarded as
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