Formation Age and Geochemical Characteristics of Supahe Granite Pluton in Longling Area, Yunnan Province: Magmatic Response to Pan-African Movement
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摘要: 泛非期中酸性岩浆事件影响的时间和空间是国内外地质学家用来确定冈瓦纳范围的重要依据和重要标志之一，也是泛非运动在青藏高原的重要表现形式，发育于云南龙陵地区的苏帕河花岗岩就是其中的酸性岩之一。产于龙陵-瑞丽断裂（简称龙瑞断裂）南东侧的苏帕河花岗岩，其主体岩性为花岗闪长岩、二长花岗岩，岩体除了发育一系列北东向狭窄的片理化带之外，主体为块状构造，少量具有弱片麻状构造。通过6个未变形的花岗岩样品的LA-ICP-MS锆石U-Pb测年，苏帕河花岗岩体侵位时代为527~519Ma，为寒武纪纽芬兰二期的酸性侵入体。通过23个未变形新鲜样品的主微量元素分析测试表明：苏帕河花岗岩体主量元素具有高硅、富碱、富铝的特征，为高钾钙碱性系列过铝质的S型花岗岩；稀土元素总量较高，ΣREE=34.69×10-6~348.20×10-6，轻重稀土分异明显，L/H=2.04~33.31，（La/Yb）N=1.31~67.65，具有强烈的负Eu异常，δEu=0.1~0.9；微量元素富集Rb、Ba、K等大离子亲石元素和Th、Ta、Ce、Hf等高场强元素，明显亏损Sr、Eu、Ti等，主微量元素均显示岩浆经历了分离结晶作用，且发生了斜长石的分异作用。Sr-Nd同位素特征表明，苏帕河花岗岩来源于中晚元古代的地壳部分熔融，并可能存有地幔物质的混入。构造环境判别图解显示出苏帕河花岗岩形成于碰撞构造环境，为泛非运动过程中泛非洲大洋闭合、碰撞的产物。Abstract: Acid magmatic events are the important basis used by geologists to delineate the area of Gondwanal and also an important manifestation of Pan-African movement in qinghai-tibet plateau. The Supahe Granite,southeast side of Longling-Ruili fault (referred to as Longrui fault), developed in Longling Area, Yunnan Province, is one of the acid plutons,mainly composed of granodiorite and monzogranite.The main pluton is of massive structure and weak gneissic structure with a series of NE trending narrow ductile deformation zones.LA-ICP-MS zircon U-Pb dating of 6 undeformed granite samples shows that the emplacement age of Supahe Granite is 527-519 Ma, being an acidic intrusion of the 2nd stage of Cambrian Newfoundland.The major and rare elements from 23 fresh undeformed samples show that Supahe granite is characterized by high silicon, alkali and aluminum.It belongs to S-type peraluminous granite with high-K, calc alkaline series.The total REE content is relatively high, with ΣREE=34.69×10-6~348.20×10-6, and the difference of light and weight rare earth elements is obvious, L/H=2.04~33.31, (La/Yb)N=1.31~67.65. It has the strong negative Eu anomaly with δEu=0.1~0.9. The trace elements are enriched in ion lithophile elements such as Rb, Ba, K and other LILE elements of Th, Ta, Ce, Hf, etc. It is obviously depleted in Sr, Eu, Ti, etc.The composition of major and rare elements shows that the magma has undergone fractional crystallization and differentiation of plagioclase.The isotopic characteristics of 87Sr/86Sr, 143Nd/144Nd indicated that the Supahe granite was originated in the partial melting of the late-Proterozoic crust, with some mantle material mixed in it. The discriminant diagram of tectonic environment showed that the Supahe granite formed in the collision tectonic environment, being the result of the closure and collision of Pan-African ocean in the Pan-African movement.
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