东天山马庄山金矿区赋矿石英斑岩的岩石成因和构造背景:元素地球化学、U-Pb年代学和Sr-Nd-Hf同位素约束

Petrogenesis and tectonic setting of the quartz porphyry in Mazhuangshan gold deposit,Eastern Tianshan Orogen: Evidence from geochemistry,zircon U-Pb geochronology and Sr-Nd-Hf isotopes

马庄山金矿位于东天山东段,矿区出露有大量的石英斑岩,总体上呈北东走向,沿矿区中部断裂侵入。石英斑岩呈灰白色,具斑状结构,斑晶主要为石英,少量正长石和斜长石,基质为长英质和少量暗色矿物。石英斑岩的Na_2O+K_2O含量为3. 55%～9. 67%,Al_2O_3含量(10. 37%～14. 28%),K_2O/Na_2O比值为2. 57～66. 5,显示富碱高钾、过铝质特征;稀土含量中等(65×10^(-6)～161×10^(-6)),轻重稀土分异中等([La/Yb]N=6. 15～12. 5),相对富集轻稀土元素、具弱的负Eu异常。微量元素总体富集Rb、K、Th、U和Pb,亏损Sr、Ba、P、Nb和Ti等元素,与活动大陆边缘弧火成岩的微量元素特征一致。马庄山石英斑岩的SiO_2与P_2O_5呈现负相关关系,Y、Th与Rb均呈现正相关的关系,说明研究区的次火山岩属于I型花岗岩类。对马庄山石英斑岩内岩浆锆石进行LA-ICP-MS U-Pb定年,获得一致曲线年龄为315. 4±0. 6Ma(MSWD=0. 67),加权平均年龄为316. 0±2. 0Ma(MSWD=0. 23),将矿区内以石英斑岩为主的次火山岩侵位时间限定在314～318Ma。石英斑岩具有较高的(87Sr/86Sr)i值(0. 7077～0. 7102),较低的εNd(t)值(-1. 62～1. 82),Nd的二阶段模式年龄(tDM2)为0. 9～1. 2Ga,εHf(t)值为-3. 2～0. 4,tDM2在1. 28～1. 51Ga之间,暗示岩石来自于下地壳物质重熔,可能有部分地幔物质加入。综合石英斑岩的地质、地球化学和年代学特征,认为东天山东段,在晚石炭世,马庄山地区处于与俯冲相关的活动陆缘弧环境。

The Mazhuangshan gold deposit is located in the eastern part of the Eastern Tianshan Orogen,where abundant quartz porphyries are widely outcropped.The quartz porphyries are EW-or NW-trending,distributing in a narrow and long belt and intruding along the fault in middle part of the mining area.It is grey-write coloured and exhibits a porphyritic texture.Quartz,K-feldspar and plagioclase are embedded in a fine-grained groundmass,which mainly comprises felsic minerals and minor dark minerals.The(Na2O+ K2O) contents of quartz porphyry samples are 3.55%~9.67%,the Al2O3 contents are 10.37%~14.28%,with the K2O/Na2O ratio values of 2.57~66.5,which show the peraluminous characteristics.The quartz porphyries have medium ∑REE contents(65 ×10^-6~161 × 10^-6) and medium REE fractionation([La/Yb]N= 6.15~12.5),with apparently enrichment of light rare earth and slightly negative Eu anomaly.The subvolcanic rocks are riched in Rb,K,Th,U,Pb elements and depleted in Sr,Ba,P,Nb and Ti elements,which is similar to the characteristic of the rocks formed in active continental margin.Quartz porphyry samples in the Mazhuangshan area show negative correlation between P2O5 and SiO2 elements,positive correlation between Y vs.Rb elements and Th vs.Rb values,which are consistent with the typical I-type granite trends.The zircons from quartz porphyries in Mazhuangshan gold deposit yielded U-Pb concordant age of 315.4 ± 0.6 Ma(MSWD = 0.67) and weighted mean zircon206 Pb/238 U age of 316.0 ± 2.0 Ma(MSWD = 0.23),which shows that quartz porphyry intruded at 314~318 Ma.Sr-Nd-Hf isotopic compositions indicate that quartz porphyry is characterized by high(87 Sr/86 Sr)ivalues(0.7077~0.7102),low εNd(t) values(-1.62~1.82),with relatively young model ages(tDM2) ranging from 0.9 to 1.2 Ga.The εHf(t) values of zircons are-3.2~0.4 and the corresponding tDM2 model ages are1.28~1.51 Ga.These features illustrate that the quartz porphyry magmas are derived from the parial melting of the lower crustal materials that involved some mantle component.Comprehensive geological,geochemical and chronological studies demonstrate that the eastern part of the Eastern Tianshan was in an active continental margin arc environment related to subduction during the Late Carboniferous.Quartz porphyry may be the product of the subduction of oceanic crust during the Late Carboniferous,which resulted in partial melting of the new lower crust that involved some mantle component and finally emplaced in the active continental margin arc environment.