湘东北新元古代过铝质花岗岩的岩石地球化学特征及其成因讨论

湘东北位于扬子板块东南缘江南造山带中段,出露的新元古代花岗岩有长三背、大围山和葛藤岭等岩体,同属于九岭岩体的一部分。湘东北新元古代花岗岩SiO2含量变化于60%~72%、CaO为0.6%~3%、Na2O为1.98%~3.72%、K2O为2.95%~4.99%之间,A/CNK>1.1,富集K、Rb、Ba等大离子亲石元素(LILE)和轻稀土元素(LREE/HREE=3.1~10.5)、Eu负异常明显(δEu=0.37~0.58),而Nb、Ta、Ti等高场强元素(HFSE)相对亏损。这些特征表明湘东北新元古代花岗岩来源于过铝质熔体,岩石类型上类似于富黑云母过铝花岗岩类(CPG),可能来源于富黑云母的变泥质沉积岩的熔融,如中元古代冷家溪群变质沉积岩等,形成于同碰撞环境,可能与陆壳加厚导致的剪切重熔有关。

PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS AND PETROGENESIS OF NEOPROTEROZOIC PERALUMINOUS GRANITES IN NORTHEASTERN HUNAN PROVINCE

The northeastern part of Hunan Province is located at the center of Jiangnan orogenic belt. The Neoproterozoic granites are mainly composed of Changsanbei, Daweishan and Getenglin intrusives. The granites belong to strong peraluminous high K calc-alkaline series. For all samples, SiO2 varies between 60% and 72%, CaO between 0.6% and 3%, Na2O between 1.98% and 3.72%, K2O between 2.95% and 4.99%, and ASI values between 1.1 and 1.5. The chondrite-normalized REE patterns are characterized by LREE enrichment with negative Eu anomalies (δEu=0.27～0.58). In the primitive mantle-normalized spidergrams, they are enriched in LIL elements and show HFSE elements depletion. These geochemical characters of granites are similar to those of the CPG, which indicates that the granitoids have been derived from peraluminous melting of metasedimentary rocks (such as Lengjiaxi Groups) enriched in biotites. High Zr (111～206μg/g), P2O5 (0.12%～0.18%) and low Al2O3/TiO2 (19～45) and FeO/MgO(1.8～3.5) ratios indicate that the granitic melts were generated under the conditions of high temperature (800～850℃) and high pressure (600 ～700MPa). Based on the tectonic development of the region, and its tectonic, petrological and geochemical characters, we suggest that the Neoproterozoic granitoids in northeastern Hunan Province have been generated in syn-collisional setting closely related to the thrust-shearing anatexis caused by crustal thickening.