新疆喀拉通克镁铁质岩体群的岩石成因研究

Petrogenesis of Kalatongke mafic rock intrusions, Xinjiang.

喀拉通克镁铁质岩体群位于准噶尔板块北缘,由13个小岩体组成。主要岩石类型为方辉橄榄岩、橄榄苏长岩、辉长苏长岩、辉长岩、淡色辉长岩、辉长闪长岩等。主要造岩矿物为贵橄榄石、普通辉石、紫苏辉石、斜长石和数量较多的褐色普通角闪石、黑云母等。主量元素分属钙碱性系列、岛弧拉斑玄武岩系列和具有MORB属性的拉斑玄武岩系列。大多数样品具有高LREE/HREE比值、富集大离子亲石元素,相对亏损高场强元素(Nb、Ta、Ti)。ε_(Nd)(t)=+6.2～+8.2,4件样品的ε_(Sr)(t)=-0.6～-36.3,另有1件样品为+36.5;(^(206)Pb/^(204)Pb)_i=17.784～18.008,(^(207)Pb/^(204)Pb)_i=15.400～15.555,(^(208)Pb/^(204)Pb)。=36.973～37.560。元素地球化学和Nd、Sr、Pb、Os、O、S同位素体系证明,岩浆仅在局部遭受了同化混染作用。原生岩浆为高镁的拉斑玄武岩浆(MgO=11.6%)。岩浆在上侵和运移过程中经历了以单斜辉石为主的暗色矿物的广泛分离结晶作用和斜长石的凝聚,导致了淡色辉长岩的广泛分布和超镁铁岩的严重缺失。岩浆源区由被消减板片交代的地幔楔物质和软流圈地幔物质组成,因而富集不相容元素和水,并因交代作用与岩浆生成的时差短,而保持放射成因同位素的亏损状态。在岩石圈根部拆沉和软流圈地幔上涌过程中生成了喀拉通克镁铁质岩体群。

Kalatongke mafic rock intrusions, which are consisted of 13 small rocks, located at the northern margin of Junggar block. They are mainly comprised of harzburgites, olivine norites, gabbronorites, gabbros, leucogabbros and gabbro-diorites. Chrysolite, augite, hypersthenes, plagioclase and lots of brown common hornblende and biotite are the main petrogenic minerals. The main elements belong to calc-alkaline series, island-arc tholeitiitic series and tholeitiitic series affine with MORB. Most samples have high LREE/HREE ratios, enrich in LILES, relatively deplete in HFSE (Nb, Ta, Ti). The epsilon(Nd)(t) value varies from 6.2 to 8.2, epsilon(Sr)(t) value varies from -0.6 to -36.3 except one is +36.5, Pb isotopic are characterized by (Pb-206/Pb-204)(i) = 17.784 similar to 18.008, (Pb-207/Pb-204)(i) = 15.400 similar to 15.555, (Pb-208/Pb-204)(i) = 36.973 similar to 37.560. Element geochemistry and Nb, Sr, Pb, Os, 0, S isotope characters indicate that the magma only experienced contamination to some extent. The MgO of primary magma constrained is 11.6%. The main dark-colored minerals are clinopyxene in the processes of the fractional crystallization and the condense of plagioclase, which lead to the dominant of leucogabbros and serious lack of ultramafic rocks. The magma source is composed of wedge matasomasised by subducting slab and aesethenosphere components, as a result, it is enriched in incompatible elements and water. The interval between the matasomatism and magma generation is short, so the depletion of radiogenic isotope is well kept. Kalatongke rock bodies are the result of the lithospheric delamination at the root and the upwelling of asthenosphere components.