Early Paleozoic high-Mg granodiorite from the Erlangping unit, North Qinling orogen, central China: Partial melting of metasomatic mantle during the initial back-arc opening
Early Paleozoic high-Mg granodiorite from the Erlangping unit, North Qinling orogen, central China: Partial melting of metasomatic mantle during the initial back-arc opening
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北秦岭造山带二郎坪单元早古生代高镁花岗闪长岩：弧后初始开启过程中交代地幔的部分熔融
DOI：
10.1016/j.lithos.2017.07.016
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发表时间：
2017-09
期刊：
影响因子：
3.5
通讯作者：
Saihong Yang
中科院分区：
文献类型：
--
作者：
Mohammed I.M. Abdallsamed;Yuan-BaoWu;Wenxiang Zhang;Guangyan Zhou;Hao Wang;Saihong Yang
本文讨论了北秦岭造山带二郎坪单元早古生代高镁花岗闪长岩的岩石学分类、成因及其构造意义。为此，对二郎坪单元坎峰沟岩体进行了原位锆石U-Pb定年、全岩地球化学及Sr-Nd-Hf-O同位素组成综合研究。坎峰沟样品LA-ICP-MS锆石定年U-Pb年龄为442.9 +/- 6.2。438.0 +/- 6.7 Ma，表明该岩体的侵位时间约为440 Ma。样品全岩地球化学组成显示中等SiO2 （60.48 ~ 64.67 wt%）和K2O （1.21 ~ 2.10 wt%）含量，Al2O3 （15.44 ~ 16.51 wt%）和Na2O （4.01 ~ 4.81 wt%）含量较高。花岗闪长岩样品MgO含量在2.30 ~ 3.44 wt%之间，Mg#值在53.35 ~ 56.66之间，为高Mg花岗闪长岩。其特征是Ba （524 ~ 1132 ppm）和Sr （684 ~ 980 ppm）含量很高，但HREE含量较低，La/Yb (N)比值为6.34 ~ 16.5,Eu/Eu* = 0.68 ~ 1.09，呈微负或弱正异常。这些证据表明，坎峰沟岩体属于类sanukit类系列。高Mg花岗闪长岩样品具有高Mg#值（53.35 ~ 56.66）、高Cr （45.8 ~ 93.3 ppm）、高Ni （282 ~ 48.2 ppm）的地幔特征，但LILE富集，表明地幔源富集。样品显示出相对较弱的放射性成因同位素组成，初始Sr-87/Sr-86比值为0.7044 ~ 0.7047,ε (Nd)(t)值为0.31 ~ 421，锆石ε (Hf) (t)值为7.3 ~ 83。锆石的δ O-18平均值为5.20 +/- 0.17‰。根据微量元素地球化学特征，认为交代介质为俯冲板块脱水产生的流体。因此，我们认为二郎坪高镁花岗闪长岩的形成过程可分为两个阶段：(1)板块流体与地幔橄榄岩的相互作用；(2)软流圈地幔上涌引起的交代地幔橄榄岩部分熔融和洋板回滚引起的弧后初始张开。
This study discussed the petrological classification, petrogenesis, and tectonic significance of early Paleozoic high Mg granodiorite from the Erlangping unit, in the North Qinling orogen. To achieve this target, we conducted integrated investigation of in situ zircon U-Pb dating, whole-rock geochemical, as well as Sr-Nd-Hf-O isotopic compositions for the Kanfenggou pluton from the Erlangping unit. LA-ICP-MS zircon dating for the Kanfenggou samples yields U-Pb ages of 442.9 +/- 6.2.and 438.0 +/- 6.7 Ma, suggesting that the pluton was emplaced at ca. 440 Ma. Whole-rock geochemical compositions of the samples display intermediate SiO2 (60.48-64.67 wt%) and K2O (1.21 to 2.10 wt%), but high Al2O3 (15.44 to 16.51 wt) and Na2O (4.01 to 4.81 wt%) contents. The granodiorite samples are characterized by elevated MgO ranging from 2.30 to 3.44 wt% and Mg# values of 53.35to 56.66, implying they are high-Mg granodiorites. They are characterized by very high Ba (524-1132 ppm) and Sr (684-980 ppm) contents, but depleted in HREE, and high (La/Yb)(N) ratios of 6.34 to 16.5 and slightly negative to weak positive Eu anomalies (Eu/Eu* = 0.68-1.09). These evidence that the Kanfenggou pluton belongs to the sanukitoid series. The high-Mg granodiorite samples exhibit a mantle signature with high Mg# values (53.35-56.66), Cr (45.8 to 93.3 ppm) and Ni (282 to 48.2 ppm) contents, but enriched in LILE, pointing to an enriched mantle source. The samples show relatively depleted radiogenic isotopic compositions with initial Sr-87/Sr-86 ratios varying from 0.7044 to 0.7047, epsilon(Nd)(t) values from 0.31 to 421, and zircon epsilon(Hf) (t) values from 7.3 to 83. The zircons have a mean delta O-18 value of 5.20 +/- 0.17 parts per thousand. Based on the trace element geochemical features, the metasomatic agent was suggested to be the fluids generated from dehydration of subducted slab. Therefore, we suggest two-stage processes for the formation of the Erlangping high-Mg granodiorites: (1) interaction between slab fluids and mantle peridotite; (2) partial melting of metasomatized mantle peridotite caused by the asthenosphere mantle upwelling and the initial back-arc opening resulted from the oceanic slab rollback.
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