豫西二郎坪满子营花岗岩体地球化学及年代学研究
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国家基础研究规划项目“973”资助项目(2009CB825003);西北大学大陆动力学国家重点实验室基金项目;国家自然科学基金资助项目(40472043)


Geochemical and zircon U_Pb chronological studies of the Manziying granite in Erlangping area, western Henan Province
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    摘要:

    对出露于豫西西峡县北部二郎坪地区的满子营花岗岩体进行了详细的岩石学、地球化学及锆石U-Pb年代学研究,重点讨论了该岩体的成因。满子营花岗岩主要由石英(40%~50%)、钾长石(25%~30%)和斜长石(15%~20%)组成,副矿物有锆石、磷灰石、磁铁矿等。地球化学分析显示,岩石总体呈高硅、富碱、高钾、高铝和低镁、低钙的特征,K2O/Na2O>1,里特曼指数σ为1.81~2.39,属高钾钙碱性系列。岩石的铝饱和指数(A/CNK)大于1.1,具有过铝质S型花岗岩的特征。稀土配分模式为右倾“V”字形态,轻重稀土分馏明显,(La/Yb)N=4.63~32.27,具有较强的负铕异常(δEu=0.38~0.63);在洋脊花岗岩标准化蛛网图上,岩石明显富集Rb、Th、K等大离子亲石元素(LILE),亏损Nb、Ta等高场强元素(HFSE),兼具同碰撞和岛弧花岗岩的特征。锆石的LA-ICP-MS原位定年获得花岗岩的结晶年龄为459.5±0.9 Ma。根据满子营花岗岩与二郎坪火山岩时、空分布的一致性和岩体性质上的相似性,认为满子营花岗岩的形成与二郎坪岛弧火山岩的形成有关,是由板块俯冲所导致的活动陆缘上陆壳物质部分熔融的产物。

    Abstract:

    The Manziying granite is located in Erlangping area, northern Xixia County, western Henan Province. This paper has studied petrographical, geochemical and zircon U-Pb chronolgical characteristics of this granite as well as its petrogenesis and geological significance. The Manziying granite mainly consists of quartz (40%~50%), orthoclase (25%~30%), plagioclase (15%~20%) with minor accessory minerals of zircon, apatite and magnetite. Geochemical analyses show that the Manziying granite has high content of SiO2 (75.01%~77.44%), Al2O3(12.15%~13.35%), K2O (3.81%~5.06%) and total alkali (K2O+Na2O=6.81%~9.34%), but relatively low MgO (0.10%~0.35%) and CaO (0.18%~0.85%). The ratios of K2O/Na2O are generally higher than 1. The Rittmann indexes are between 1.81 and 2.39, belonging to the high-K calc-alkaline series. The values of A/CNK vary from 0.99 to 1.16 with an average of 1.10, showing characteristics of peraluminous S-type granites. Trace element analyses show that the Manziying granite has relatively high REE content of 121.9×10-6~289.5×10-6, and displays right-inclined V-shape REE patterns with relatively high fractionations of LREE and HREE [(La/Yb)N=4.63~32.27)] and pronounced negative Eu anomalies (δEu=0.38~0.63) in the chondrite-normalized REE diagrams, suggesting a typical crust-derived granite. The concentration of Nb (6.6×10-6~11.5×10-6) and Ta (0.5×10-6~1.2×10-6) and the ratios of Nb/Ta (7.2~10.0) in the Manziying granite are very similar to those of crustal rocks and the typical continent-continent syn-collisional granite, also suggesting that the Manziying granite was derived from the melt of crustal rocks. The Manziying granite is rich in LILEs of K, Rb, Th and Ba, poor in HFSEs of Nb and Ta. The distinct depletion of Nb and Ta together with the depletion of Ba and enrichment of Ce relative to their neighbor elements in the ocean ridge granite-normalized trace element diagram indicate that the Manziying granite has features of both island arc and syn-collisional granite. In addition, almost all samples fall into the syn-collisional granite and island arc granite field in the trace elements diagrams of Nb-Y and Rb-Y+Nb. The cathodoluminescence (CL) investigations have revealed that zircons from the Manziying granite are generally colorless, transparent, short prismatic euhedral crystals (from 50 μm to 200 μm in length and from 50 μm to 150 μm in width) with oscillatory internal zoning. LA-ICP-MS U-Th-Pb analyses show that these zircons have comparatively high content of Th (110×10-6~1859×10-6) and U (247×10-6~1851×10-6), with Th/U ratios ranging from 0.51 to 1.24 (except for one sample, whose value is 0.14), indicating their magmatic origin. 22 spots of 18 zircon grains yield 206Pb/238U ages varying from 429 Ma to 464 Ma. All data are spread along the concordant line or in adjacent areas, in which 16 spots are concentrated in a small area, and yield a 206Pb/238U weighted average age of (459.5±0.9) Ma, implying the magmatic crystallization age.The Manziying granite has a close relationship in space with Erlangping basic volcanic rocks, which are considered to have been formed in the island arc and back arc-basin and related to the subduction of oceanic-slab. Recently, SHRIMP zircon dating gave the formation age of 466 Ma for the basic volcanic rocks, which is almost the same as the formation age of the Manziying granite reported in this paper. Considering the coherence in space and time and the similarity of geochemical features between these two kinds of rocks, conclude that the formation of the Manziying granite had close relationship with the Erlangping island arc volcanic rocks, and probably resulted from the partial melting of crustal rocks in the active continental margin caused by the plate subduction.

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郭彩莲,陈丹玲,樊 伟,等, 2010. 豫西二郎坪满子营花岗岩体地球化学及年代学研究[J]. 岩石矿物学杂志, 29(1):15~22.
GUO Cai_lian, CHEN Dan_ling, FAN Wei, et al, 2010. Geochemical and zircon U_Pb chronological studies of the Manziying granite in Erlangping area, western Henan Province[J]. Acta Petrologica et Mineralogica, 29(1): 15~22.

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