鲁西沂山地区高级深熔混合岩中存在丰富的不同时代、不同类型的岩石包体。在详细野外地质工作基础上,本文对高级深熔混合岩中的包体进行了锆石SHRIMP U-Pb定年和Hf同位素分析及全岩地球化学研究,以期探讨沂山地区高级深熔混合岩的源区性质。英云闪长质片麻岩形成时代为~2.7Ga,存在2.69Ga和2.53~2.51Ga两期基性岩浆作用,并记录了2.52~2.50Ga变质作用。角闪斜长片麻岩与黑云角闪变粒岩互层产出,它们可能为变质火山岩,其岩浆锆石年龄为2.51Ga。变质基性岩的岩浆锆石εHf(t)为0~+8.1,一阶段Hf模式年龄为2.83~2.54Ga;变质火山岩的岩浆锆石εHf(t)为-1.2~+5.8,一阶段Hf模式年龄为2.89~2.71Ga。结合前人研究,这套高级深熔混合岩的源区组成包括了新太古代早期的英云闪长质片麻岩和变质辉长岩,新太古代晚期的石英闪长质片麻岩、变质火山岩及斜长角闪岩等岩石类型。~2.5Ga基性岩浆作用在鲁西地区发育广泛,为新太古代晚期深熔作用及高级深熔混合岩和壳源花岗岩的形成提供了热源。;There are abundant enclaves of different ages and types within diatexite in the Yishan area, western Shandong. Based on field work, this paper carried out zircon SHRIMP U-Pb dating and Hf isotope analysis and whole-rock geochemical studies on enclaves in the diatexite. A tonalitic gneiss enclave has a formation age of ~2.7Ga. Two episodes of Neoarchean mafic magmatism are recognized at 2.69Ga and 2.53~2.51Ga, respectively, and they have metamorphic ages of 2.52~2.50Ga. Hornblende-plagioclase gneiss was interbedded with fine-grained biotite-hornblende gneiss, and these two types of gneisses are considered to be meta-volcanic rocks. The hornblende-plagioclase gneiss has a magmatic zircon age of 2.51Ga. The magmatic zircon from the meta-gabbro has εHf(t) values of 0~+8.1 with tDM1 of 2.83~2.54Ga, whereas the magmatic zircon from the hornblende-plagioclase gneiss has εHf(t) values of -1.2~+5.8 with tDM1 of 2.89~2.71Ga. Combined with previous work, it is speculated that the source of the diatexite includes Early Neoarchean tonalitic gneiss and meta-gabbro, and Late Neoarchean quartz diorite gneiss, meta-volcanic rock and amphibolite.~2.5Ga mafic magmatism is widely developed in western Shandong, which provides the heat source for the Late Neoarchean anataxis, resulting in the formation of diatexite and crustally-derived granite.
The mainland of China is composed of the North China Craton, the South China Craton, the Tarim Craton and other young orogenic belts. Amongst the three cratons, the North China Craton has been studied most and noted for its widely-distributed Archean basement rocks. In this paper, we assess and compare the geology, rock types, formation age and geochemical composition features of the Archean basements of the three cratons. They have some common characteristics, including the fact that the crustal rocks prior to the Paleoarchean and the supracrustal rocks of the Neoarchean were preserved, and Tonalite-Trondhjemtite-Granodiorite (TTG) magmatism and tectono-magmatism occurred at about 2.7 Ga and about 2.5 Ga respectively. The Tarim Craton and the North China Craton show more similarities in their early Precambrian crustal evolution. Significant findings on the Archean basement of the North China Craton are concluded to be: (1) the tectonic regime in the early stage (>3.1 Ga) is distinct from modern plate tectonics; (2) the continental crust accretion occurred mostly from the late Mesoarchean to the early Neoarchean period; (3) a huge linear tectonic belt already existed in the late Neoarchean period, suggesting the beginning of plate tectonics; and (4) the preliminary cratonization had already been completed by about 2.5 Ga. Hadean detrital zircons were found at a total of nine locations within China. Most of them show clear oscillatory zoning, sharing similar textures with magmatic zircons from intermediate-felsic magmatic rocks. This indicates that a fair quantity of continental material had already developed on Earth at that time.
Abstract In recent years diamonds and other exotic minerals have been recovered from mantle peridotites and high‐Cr chromitites of a number of ophiolites of different age and different tectonic environments. Here we report a similar collection of minerals from the Sartohay ophiolite of Xinjiang Province, western China, which is characterized by having high‐Al chromitites. Several samples of massive podiform chromitite with an aggregate weight of nearly 900 kg yielded diamonds, moissanite and other highly reduced minerals, as well as common crustal minerals. Thus far, more than 20 grains each of diamond and moissanite have been recovered from heavy mineral separates of the chromitites. The diamonds are all 100–200 μm in size and range in color from pale yellow to reddish‐orange to colorless. Most of the grains are anhedral to subhedral octahedra, commonly with elongate forms exhibiting well‐developed striations. They all display characteristic Raman spectra with shifts between 1325 cm −1 and 1333 cm −1 , mostly 1331.51 cm −1 or 1326.96 cm −1 . The moissanite grains are light blue to dark blue, broken crystals, 50–150 μm across, commonly occurring as small flakes or fragments. Their typical Raman spectra have shifts at 762 cm −1 , 785 cm −1 , and 966 cm −1 . This investigation extends the occurrence of diamonds and moissanite to a Paleozoic ophiolite in the Central Asian Orogenic Belt and demonstrates that these minerals can also occur in high‐Al chromitites. We conclude that diamonds and moissanite are likely to be ubiquitous in ophiolitic mantle peridotites and chromitites.
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