新疆东准噶尔黄羊山碱性花岗岩体的锆石U-Pb年龄和岩石成因

ZIRCON U-Pb AGE AND PETROGENESIS OF THE HUANGYANGSHAN ALKALINEGRANITE BODY IN EAST JUNGGAR,XINJIANG

新疆东准噶尔卡拉麦里构造带分布有多种类型花岗岩,主要包括花岗闪长岩、二长花岗岩、碱长花岗岩和碱性花岗岩。对其中最大的碱性花岗岩体(黄羊山岩体)进行定年和地球化学组成研究,结果显示,岩体的年龄约为305 Ma,岩石富含钠质角闪石(钠闪石、钠铁闪石)和少量霓石,具低铝、富碱、贫钙镁及低铁的主量元素特征,其微量元素明显富集Rb、Th等大离子亲石元素和Zr、Hf等高场强元素而强烈亏损元素Ba、Sr、Eu,稀土元素配分模式呈典型的平坦‘V’字型。这些矿物学和地球化学组成特征表明黄羊山碱性花岗岩属于典型的A型花岗岩。认为幔源岩浆高度分异、麻粒岩相残留岩及I型英云闪长质-花岗闪长质岩石部分熔融等A型花岗岩成岩模式不能解释黄羊山碱性花岗岩的成因。根据岩石的εNd(t)=+5.9^+6.5,比当时洋壳的εNd(t)值低3~4个ε单位,认为形成该岩体的岩浆可能来源于花岗闪长质岩浆的分异结晶,而花岗闪长质岩浆则主要是玄武质洋壳和少量大洋沉积物(约5%)部分熔融的产物。

Several types of granites mainly including granodiorites, monzogranites, alkali feldspar granites and alka-line granites, are distributed in the Karamaili tectonic belt of East Junggar, Xinjiang. In this paper, the geochrono-logical and geochemical results for the largest alkaline granite body （the Huangyangshan body） are reported. Zircon U-Pb dating by LA-ICPMS shows that the alkaline granites from Huangyangshan body were emplaced at about 305 Ma. These alkaline granites contain sodic amphibole （riebeckite and/or arfvedsonite） and minor aegirine, have high contents of alkalis but low abundances of Al2O3, CaO, MgO, FeO, are characterized by significant enrichment in LILE （such as Rb, Th）, HFSE （such as Zr, Hf） and strong depletion in Ba, Sr, Eu, with fiat V-shaped REE patterns. All these mineralogical and geochemical features indicate that the alkaline granites from the Huangyangshan body are actually typical A-type granites. However, the A-type granites here cannot be explained by previous petrogenetic models including extensive fractional crystallization from mantle-derived magmas, and partial melting of granu-litic residue or I-type tonalitic to granodioritic rocks. On the basis of the εNd（t） values of ＋5.9 to ＋6.5 for the al-kaline granites, 3 -4 epsilon units lower than that of the oceanic crust at 305 Ma in the studied region, it is suggested that the alkaline granites were likely formed by fractional crystallization of granodioritic magmas, which were probably produced by partial melting of the remnant basaltic oceanic crust plus a few （ about 5 % ） marine sediments.