湘东北金井地区花岗岩成因及地球动力学暗示:岩石学、地球化学和Sr-Nd同位素制约

Constraints of petrography, geochemistry and Sr-Nd isotopes on the Jinjing granitoids from northeastern Hunan province, China: implications for petrogenesis and geodynamic setting.

出露于湘东北地区的燕山期花岗质岩岩体,呈岩基或岩株状广泛侵位于中元古界冷家溪群浅变质碎屑沉积岩中, 部分岩体为白垩纪红层覆盖。文章详细研究了金井岩体的岩石学、地球化学及Sr-Nd同位素特征,并与其围岩冷家溪群进行了对比。该岩体具有较高的SiO2(71．44-74．31wt．％)、K2O(3．86-4．98wt．％)和Na2O(3．24-3．84wt．％),以及较低的 FeO+Fe2O3+MgO+TiO2(均小于3％),结合高含量的Al2O3(13．25-14．89wt．％)及高ASI值(普遍大于1．1),为典型的强过铝质(SP)高钾钙碱性花岗岩。金井花岗岩富硅、碱和高Sr、Ba丰度、高LILE／HSFE和LREE／HREE比值,低镁铁质及REE 含量,表明金井岩体为高度分异的花岗岩,其源区的残留物中黑云母含量高。REE配分图和原始地幔多元素标准化图谱上 Rb、Th、U、K和LREE相对富集,而Ba、Sr和高强场元素(Nb、Ta、P和Ti)相对亏损,Eu可变的负异常(δEu:0．53-0．78),表明存在角闪石、黑云母、堇青石、磷灰石和钛铁矿等分异演化的影响,类似于同碰撞型花岗岩地球化学特征。高Rb／Sr(3-6)、低 Sr／Ba(0．2-0．7)、高P2O5含量(0．09-0．15wt．％)、低Al2O3／TiO2比值(40-70<100)、Zr含量(100-150ppm)以及FeO／MgO 等特征表明该过铝质花岗岩浆可能是在高温(±800℃)高压(-8kbar)奈件下、经中下地壳变质沉积岩黑云母脱水熔融形成。 CaO／Na2O的差异(>0．4和<0．2两组)以及Nd同位素的不同(高Nd和低Nd两组),类似于围岩冷家溪群砂质和泥质沉积岩特征,以及高Sr[(87Sr／86Sr)i=0．72204-0．72540]、低Nd[εNd(145 Ma)=-10--12]等均表明花岗质岩浆主要起源于中元古代冷家溪群变质砂岩(高Ca)和变质泥岩(低Ca)的部分熔融或中下地壳物质的熔融,但有少量地幔物质的加入。结合印支期以来华南及湘东北地区地球动力学演化史,作者认为金井燕山期花岗岩形成于陆内碰撞造山晚期,是160-140Ma 陆内俯冲(A型俯冲)碰撞造山作用增温减压体制达到鼎盛阶段的产物,标志湘东北地区由此全面转入陆内伸展阶段。

The Yanshanian granitoids occurred widely in northeastern Hunan Province, China and intruded into the Mesoproterozoic Lengjiaxi group. The granitoids in Jinjing area belong to strong peraluminous high K calc-alkaline series, which show high contents of SiO2 （ average at 72. 8% ） and Na2O＋K2O （7.16-8.74%）, and high values of ASI （generally 〉1.1）. Based on content of CaO,the granitoids can be divided into high Ca-type and low Ca-type. The ehondrite-normalized REE patterns are characterized by LREE enrichment with variable degree of negative Eu anomalous sizes （ δEu = 0. 53 - 0. 78, average at 0.63 ）. The Primitive mantalnormalized spidergrams show that they are enriched in LILE and depleted in HFSE. These geochemical characteristics are similar to those of the Mesoproterozoic Lengjiaxi Group except for the remarkably depletion in Sr, indicate that the granitoids have been derived from the Mesoproterozoic Lengjiaxi Group, and that abundant biotites are still in the residue. The εNd （ 140 Ma） values （ - 10 - - 12）and the initial ^87Sr/^86Sr ratios （0. 72204 - 0. 72540） further support that the Jinjing pluton has a derivation from partial melting of metasandstone （high Ca） and metamudstonc （low Ca） , but significant input of minor mantle material into the provenance cannot be ruled out, due probably to K-rich basaltic magmatic underplating resulted by lithosphere delamination and upwelling of hot asthenosphere. High Rb/Sr （ 3 - 6 ） and high P2 O5 contents （ 0.09 - 0.15 wt. % ） , low Sr/Ba （ 0.2 - 0.7 ） , AlO3/TiO2 ratios （ 40 - 70 〈 100）, low Zr contents （ 100- 150ppm）and FeO/MgO ratios indicate that the granitic melts were generated under high temperature（ ±800℃ ）, high pressure （ - 8kbar） and water-undersaturated conditions through biotite-dehydration melting within middle to lower crust. Based on the tectonic development of South China, we suggest that the Yanshanian granitoids in Jinjing area have been generated in intra-continental syn-collision setting, hinting that the tectonic regime has been transformed from compression to extension across the Mesozoic-Cenozoic transition in northeastern Hunan Province.