Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling
Petrogenesis of Late Triassic mafic enclaves and host granodiorite in the Eastern Kunlun Orogenic Belt, China: Implications for the reworking of juvenile crust by delamination-induced asthenosphere upwelling
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中国东昆仑造山带晚三叠世基性包体和花岗闪长岩的岩石成因：拆沉引起的软流圈上升流对新生地壳改造的意义
DOI：
10.1016/j.gr.2020.02.012
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发表时间：
2020-08
影响因子：
6.1
通讯作者：
Zhao Xu
中科院分区：
文献类型：
--
作者：
Zhou Hongzhi;Zhang Daohan;Wei Junhao;Wang Dazhao;Santosh M.;Shi Wenjie;Chen Jiajie;Zhao Xu
东昆仑造山带广泛分布着富含镁铁质微粒包体的晚三叠世花岗岩类。本文介绍了鄂尔多斯东部鄂拉山地区虎达岩体的矿物化学、锆石U-Pb年龄和L-Hf同位素、全岩化学和S-Nd同位素组成。这些岩石含有继承的(中-新元古代)和捕虏晶(约240 Ma)的锆石颗粒，获得了明显的较老年龄，而来自MME和花岗闪长岩的岩浆锆石获得了大约224 Ma的加权平均年龄，这被解释为它们的结晶年龄。微环境中二氧化硅含量较低，但二氧化钛、三氧化二铁、氧化钙、氧化镁和锰氧化物含量较高，具有较高的镁#值(48-54)和100MnO/(锰+三氧化二铁+三氧化二铁)比值(1.2-1.6)。它们与寄主花岗岩具有相同的锶、钕、氢同位素组成。结合岩石学证据，我们认为MME是岩浆演化晚期压力猝灭形成的同源堆积体，来自寄主花岗闪长岩的同一母岩浆，而不是岩浆混合成因。花岗闪长岩为钙碱性-高钾钙碱性、准铝质I型花岗岩。它们具有相对较低的SiO_2和MnO含量，但具有较高的MgO、Al_2O_3、CaO和TFe_2O_3含量，Mg#值在45~50之间。它们富含轻稀土元素(LREE)和大离子亲石元素(LILEs)，如Rb、Th、K和Pb，贫P和高场强元素(HFSE)，包括Nb、Ta和Ti。这些岩石具有轻微的Eu负异常和低的Sr/Y和La/Yb比值。结合一些斑晶的环向化学演化特征，较高的初始锶同位素(0.70888~0.70912)、较低的全岩εNd(T)(−5.6~−6.0)和锆石εHf(T)(−3.3~−0.1)，以及较低的Nb/Th(0.11~0.26)和Ta/U(0.53~0.68)，我们认为花岗闪长岩浆来源于下地壳。考虑到它们相对年轻的两阶段模式年龄(分别为1.42-1.49和1.13-1.42)以及与EKOB之下年轻地壳的微量元素特征，我们认为年轻的下地壳是花岗闪长岩的主要源岩。根据我们的资料和区域地质证据，我们认为年轻地壳的部分熔融是与拆沉有关的软流圈地幔上涌的结果。后一过程导致下地壳大量熔融，在EKOB产生了一次重大的晚三叠世岩浆爆发事件。
Late Triassic granitoids containing abundant mafic microgranular enclaves (MMEs) occur widely in the Eastern Kunlun Orogenic Belt (EKOB). In this work, we present mineral chemistry, zircon U-Pb ages and L-Hf isotopes, whole-rock chemistry and S-Nd isotope compositions of the MMEs and host granodiorite from the Huda pluton in the Elashan area within the easternmost domain of the EKOB. These rocks contain inherited (Meso- to Neoproterozoic) and xenocrystic (ca. 240 Ma) zircon grains that yield apparent older ages, whereas the magmatic zircons from MMEs and granodiorite yield similar weighted mean ages around 224 Ma, which are interpreted as their crystallization ages. The MMEs have low SiO2but high TiO2, TFe2O3, CaO, MgO and MnO concentrations with relatively high Mg#values (48–54) and 100MnO/(MnO + MgO + TFe2O3) ratios (1.2–1.6). They display identical Sr-Nd-Hf isotope compositions to the host granite. Combined with petrological evidence, we suggest that the MMEs are cognate cumulates that formed by pressure quenching during the late stage of magma evolution from the same parental magma of the host granodiorite, rather than a magma mixing origin. The granodiorite is calc-alkaline to high-K calc-alkaline, metaluminous I-type granite. They show relatively low SiO2and MnO, but high MgO, Al2O3, CaO and TFe2O3contents with Mg#values of 45–50. They are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), such as Rb, Th, K and Pb, and are depleted in P and high field strength elements (HFSE) including Nb, Ta and Ti. These rocks display slightly negative Eu anomalies and low Sr/Y and La/Yb ratios. Together with the rim-ward chemically evolved nature of some phenocrysts, the comparatively high initial Sr isotope (0.70888–0.70912), low whole-rockεNd(t) (−5.6 to −6.0) and zirconεHf(t) (−3.3 to −0.1) values, and low Nb/Th (0.11–0.26) and Ta/U (0.53–0.68) ratios, we suggest that the granodiorite magma was sourced from the lower crust. Considering their comparatively young two-stage Nd and Hf model ages (1.42–1.49 Ga and 1.13–1.42 Ga, respectively) and same trace element character with the juvenile crust beneath the EKOB, we interpret the juvenile lower crust as the dominant source rocks for the granodiorite. Based on our data and regional geological evidence, we suggest that the partial melting of juvenile crust resulted from delamination-related asthenosphere mantle upwelling. The latter process resulted in extensive melting of the lower crust, producing a major Late Triassic magmatic flare-up event in the EKOB.
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