准噶尔—天山—北山蛇绿岩:对中亚造山带西南缘洋陆格局演化的制约

准噶尔、天山和北山52个蛇绿岩的地质特征、地球化学性质和同位素年代学资料系统集成研究表明它们可以分为14条蛇绿(混杂)岩带。绝大多数蛇绿岩呈"岩块+基质"的混杂岩型式沿重要断裂带(构造线)线状分布,少数蛇绿岩以构造岩片叠置方式面状产出。混杂岩的基质有蛇纹岩(碳酸盐化蛇纹岩)和糜棱岩化细碎屑岩两类,岩块既有地幔橄榄岩、基性杂岩和基性火山岩等蛇绿岩组分,也有其它非蛇绿岩组分岩石。堆晶岩出露局限,典型席状岩墙群没有发育。这些蛇绿岩可归类为SSZ(Supra-Subduction Zone)和MORB(Mid-Ocean Ridge)两种类型,前者玄武岩具大离子亲石元素(LILE)富集和高场强元素(HFS)亏损特征,后者不显示该特点;洋岛玄武岩(OIB)既可出现在SSZ型蛇绿混杂岩中,也可为MORB型的组成部分;SSZ型蛇绿混杂岩辉长岩和玄武岩比MORB型具有相对更富集的Sr-Nd同位素组成,但部分形成于弧后(间)盆地的SSZ型蛇绿岩与MORB型一致,具有近亏损地幔的Sr-Nd同位素组成。已确认的最老蛇绿岩为西准噶尔572 Ma玛依勒,次之为北山542~527 Ma月牙山—洗肠井和西准噶尔531 Ma唐巴勒,最年轻蛇绿岩为325 Ma北天山巴音沟和321 Ma北山芨芨台子。根据蛇绿岩证据,结合近年来中亚造山带古地磁、岩浆岩、高压—超高压变质岩和构造地质方面的进展,可以推断埃迪卡拉纪末期—早寒武世,古亚洲洋已达到一定规模宽度,发育洋岛和洋内弧;早古生代时期,多岛洋格局发育至鼎盛期,一系列弧地体分别归属哈萨克斯坦微陆块周缘的科克切塔夫—天山—北山线性弧、成吉思弧、巴尔喀什—西准噶尔弧体系和西伯利亚南部大陆边缘弧体系;晚古生代时期,古亚洲洋于石炭纪末期闭合,增生杂岩和弧地体组成哈萨克斯坦拼贴体系和蒙古拼贴体系两个巨型山弯构造。

The Junggar,Tianshan and Beishan ophiolites:Constraint on the evolution of oceanic and continental framework along the southwestern margin of the Central-Asian Orogenic Belt

The systemic and comprehensive study on geological characteristics, geochemical features and isotopic chronologies of the 52 ophiolites occurring in the Junggar, Tianshan and Beishan orogenic belts indicates that they can be assigned as 14 ophiolitic(mélange)belts. Most of ophiolites are exposed along the important fault belts(or tectonic lines)in a‘block-in-matrix’mélange style, while minor ophiolites occur as dispersed thrust slice piles. The matrix of ophiolitic mélanges has two types, the serpentinite(carbonated serpentinite)and the mylonitic fine-grained clastic rock. The block contains the ophiolitic componnet consisting of mantle peridotites, basic complexes and basic volcanics, furthermore, it also includes some other rocks with non-ophiolitic affinities. The cumulate occurs locally at a few ophiolite suits, and the typical sheeted dike is absent. All the ophiolites can be classified as the SSZ and MORB types.The basalts of the former have a LILE-enriched and HFSE-depleted characteristic and those of the latter type do not show this geochemical feature. The oceanic island basalt(OIB)can occur not only in the SSZ-type ophiolitic mélange but also in the MORB-type one. The gabbro and basalt of SSZ-type ophiolitic mélanges generally have more enriched Sr and Nd isotopic compositions, compared with those of MORB-type ones. However, some of the SSZ-type ophiolitic mélange, which was formed in a back arc or an intra-arc basin, have Sr and Nd isotopic compositions almost similar to those of the depleted mantle, resembling to the MORB-type ophiolite. The oldest identified ophiolite is the 572 Ma Mayile ophiolite exposed in western Junggar. And, the 542~527 Ma Yueyashan-Xichangjin ophiolite in Beishan and the 531 Ma Tangbale ophiolite in western Junggar take the second place. The youngest ophiolite is the 325 Ma Bayingou ophiolite in northern Tianshan mountains and the 321 Ma Jijitaizi ophiolite in Beishan Mountains. According to the result from ophiolites, integrated with the recent progresses on the paleomagnetism, magmatic rocks, high/ultra-high pressure metamorphic rocks and tectonics of the Central-Asian Orogenic Belt, we can speculate that the Paleo-Asian Ocean had extend to a considerable width and was consisted of oceanic islands and intra-oceanic arcs at the end of Ediacaran to the earliest of Cambrian. The archipelagic ocean reached a period of great prosperity in the Early Paleozoic time. A series of arc terranes were assigned as the Kokchetav-Tainshan-Beishan linear, the Chingiz and the Balkash-western Junggar arc system around the Kazakhstan micro-continent and the continental arc system in the southern margin of the Siberian Craton. In the Late Paleozoic time, the Paleo-Asian Ocean was closed at the end of Carboniferous, resulting in the formation of two huge orocline tectonics,the Kazakhstan and the Mongolian collages, accompanying the amalgamation of accretionary complexes and arc terranes.