王安镇岩体岩石地球化学特征及成因探讨

Petrological and geochemical characteristics of the Wanganzhen complex and discussion on its genesis.

王安镇岩体是我国东部太行山-大兴安岭中生代构造岩浆带中规模最大、岩石类型最多的代表性岩体之一,该岩体由早期、主期和晚期三期岩石组成。本文通过对王安镇岩体三期岩石主量元素、稀土元素、微量元素和Sr、Nd、Pb同位素的系统研究,发现该岩体主期岩石(花岗闪长岩、二长花岗岩和少量石英闪长岩)具有与埃达克岩类似的独持的岩石地球化学特征:SiO_2≥56％,Al_2O_3≥15％,低Y(4.20～12.68μg/g)而高Sr(518～861μg/g)和Sr/Y值(60.19～178.10),LREE强烈富集(La/Yb)_N=34.06～76.91,HREE强烈亏损Yb=0.44～1.00,Sr、Eu无明显异常,高场强元素(如Nb,Hf和Ti)相对亏损,I_(Sr)值为0.7060,ε_(Nd)(t)均为负值(-16.29～-14.27),ε_(Sr)(t)均为正值(+23.7～+25.0),应属于埃达克质岩,但又与俯冲板片熔融形成的埃达克岩的地球化学组成明显不同,推测可能是由于晚中生代区域内热地幔物质是“蘑菇云”状上涌,使热侵蚀面抬升到地壳底部,导致玄武质下地壳在高压下发生部分熔融的产物。而早期和晚期岩石则属于非埃达克质岩,早期为基-中性岩石(角闪石岩、辉长岩和闪长岩),富Mg、Fe,LREE富集,Eu呈正异常,富集相容元素而亏损不相容元素,I_(Sr)=为0.7056,ε_(Nd)(t)=-16.72～-10.92,推测是华北地台岩石圈富集地幔部分熔融的产物,

The Wanganzhen complex is one of representative example of Taihang-Da Hinggan tectonomagmatic belt in eastern China, which has the largest scale and the most rock types. It consists of three-stage rocks. According to the major element, trace element, REE and Nd,Sr,Pb isotopic characteristics of the Wanganzhen complex reported in this paper, the main stage rocks (granodiorite, adamellite and few quartz diorite) have the similar petrological and geochemical characteristics to those of adakite with SiO(2)greater than or equal to58%, Al(2)O(3)greater than or equal to15 %, low Y content (4.20similar to12.68mug/g), high Sr content (518similar to861mug/g) and Sr/Y ratios (60.19similar to178.10), LREE enrichment with (La/Yb)(N)=34.06similar to76.91, HREE depletion with Yb=0. 44similar to1.00, no obviously Eu anomaly (Eu/Eu-* = 0.83similar to1.03), and relative depletion in the high field strength elements (such as Nb, Hf, Ti), I-Sr=0.7060, negative epsilon(Nd)(t) value(-16.29similar to14.27)and positive epsilon(Sr)(t) value (+23.7similar to+25.0), but obviously different from those of subduction-related adakites. These adakite-like rocks were most likely derived from the partial melting of basaltic lower crust in the thickened crust resulted from hot mantle material upwelling as several mushroom cloud and the thermally altered ground uplifting to the bottom of continental crust during late Jurassic period. But the early and the late stage ones belong to non-adakites. The early basic- to intermediate- instrusive rocks (hornblendite, gabbroite and diorite) are high Mg and Fe, LREE enrichment, Eu positive anomaly, enriched in compatible elements and low incompatible elements, I-Sr = 0.7056, epsilon(Nd)(t)=-16.72similar to10.92, which are the products of partial melting of subcontinental lithospheric enrich mantle and probably represent the underplating basaltic magma below the thickened continental crust during early Mesozoic era. The late alkaline intermediate- to acid-intrusive rocks (alkali-granite, quartz syenite and quartz monzonite) are characteristic of high K and Na, alkaline series and high SigmaREE, negative Eu anomaly, low Sr content, high Y and Yb, negative Sr anomaly, I-Sr = 0.7062, epsilon(Nd)(t)=-16.25,suggesting that the formation of adakite-like rocks of the main stage result in the lithospheric delamination and cause asthenosphere to upwell to the bottom of thinned continental crust, so the rocks of the late stage are formed by the partial melting of the materials in the crust-mantle transformation belt even in the middle crust. In this paper the Rb-Sr isochronal age of granodiorite (145.7 +/- 5.3Ma) is reported. Based on the Rb-Sr isochronal age of granite and granidiorite, the main body of the Wanganzhen complex was probably formed in late Jurassic period between 137.8Ma and 145.7Ma.