东天山卡拉塔格钠质火山岩岩石学、地球化学及成因

Petrology,geochemistry and genesis of the Na-rich volcanic rocks of the Kalatag area,eastern Tianshan.

东天山卡拉塔格早泥盆世卡拉塔格组火山岩主要岩性为海相富钠质的安山岩、流纹岩和英安岩，夹少量的钠质玄武安山岩和钠质玄武岩以及正常系列的玄武安山岩。钠质火山岩的特征是：Na2O=1．69～6．38％，Na2O／K，O〉1，多在2～100间：无论是斑晶还是基质，斜长石均为富钠长石；酸性岩石斑晶主要是钠长石和石英，中基性岩石斑晶主要是钠长石（少量更长石）：酸性岩以斑状结构、霏细结构、球粒结构为主，而中基性岩以交织结构、玻晶交织结构为主。各类岩石全岩化学分析计算结果均含标准矿物Q、Hy、Hm，一部分含Di或C，在TAS图上全都投于亚碱性区；∑REE=25、96-96．11ppm，LaN／YbN=1．68-5．32，Eu／E^＊=0．46-1．25，LaN／SmN=1．01-2．95；强不相容元素适度富集，Nb和Ta强烈亏损，部分样品Cs、Rb、K、Zr和Hf适度亏损。这些特征表明，该区钠质火山岩的原始岩浆可能来自富钠俯冲洋壳熔体与地幔楔岩石反应的产物部分熔融。这样的岩浆形成机制与前人有关卡拉塔格是早泥盆世岛弧的认识相一致。

Volcanic rocks of the Lower Devonian Kalatag formation in the Kalatag region, eastern Tianshan are dominated by Na-rich andesite, rhyolite and dacite rocks with less Na-rich basaltic andesite and basalt and minor basaltic andesite rocks. These rocks are characterized by high Na2O content （mostly 〉 3% ） and N2O/K2O ratio （mostly 〉 2% ）. A majority of the plagioclase both as phenocrysts and in the matrix of the Na-rich rocks is albite, and oligoclase is found occasionally. The phenocryst of acidic rocks consists mainly of albite and quartz, while that in mafic and intermediate rocks consist of albite with oligoclase being occasionally seen. Acidic rocks are characterized by porphyritic, microlitic and spherulitic textures, while mafic and intermediate rocks features pilotaxitic and hyalopilitic textures. All of the whole-rock CIPW calculations contain Q, Hy and Hm, but some samples contain normative Di and C. All the plots on the TAS diagram are located in the subalkline area. REE behaviour in the rocks are characterised by ∑ REE = 25.96 - 96.11 ppm, LaN/YbN = 1.68 -5.32, Eu/Eu^＊ =0.46 -1.25, and LaN/SmN = 1.01 -2.95. The rocks are moderately enriched in most highly incompatible elements, strongly depleted in Nb and Ta, and moderately depleted, in some samples, in Cs, K, Rb, Zr and Hf. These characteristics demonstrate that these Na-rich volcanic rocks were originated by partial melting of a source in the mantle wedge, which had been reacted upon by a sodic magma resultant from partial melting of the subducting slab. Such a mechanism of petrogenesis is consistent with previous explanation that the Kalatag region is a volcanic island arc during the early Devonian.