苏北东海超镁铁岩研究

ULTRAMAFIC ROCKS IN THE DONGHAI AREA,NORTHERN JIANGSU

苏北东海超镁铁岩分为南、北两带。北带以变质橄榄岩为主，南带以洋壳堆积超镁铁岩为主，是“肢解的蛇绿岩”。推测蛇绿岩形成于中—新元古代扬子古大陆板块北缘苏北—胶南地体与胶北地体间的洋盆环境。变质橄榄岩为高温超高压下原始地幔岩经中等程度熔融后的残余地幔橄榄岩。堆积超镁铁岩是原始地幔岩部分熔融或岩浆结晶分离的产物，富含石榴石，与榴辉岩块伴生，是在中—新元古代由洋壳向南俯冲消减过程中形成的相对低温超高压变质岩。它们遭受强烈破碎，被挤压到东海岩群下部不同构造层位中，成为东海俯冲带混杂岩中的深源岩块。

Ultramafic rocks in Doughal, northern Jiangsu, fall into two belts. The northern beltconsists dominantly of metaperidotite,while the southern belt ultramafic cumulates of oceanic crust. They are dismembered ophiolites. It is inferred that the ophiolites originated in an ocean basin between the northern Jiangsu-Jiaonan terrane and the Jiaobei terrane on the northern margin of the Meso-to Neoproterozoic Yangtze paleocontinental plate. Metaperidotite isrepresented by relict mantle peridotite resulting from moderate degrees of fusion of primarypyrolite at high P/T. Ultramafic cumulates are the product of partial melting of primary pyrolite or magmatic crystallization differentiation,enriched in garnet and associated with eclogite,and are relatively low-temperature, ultrahigh-pressure metamorphic rocks formed in theprocess of southward subduction of oceanic crust in the Meso-to Neoproterozoic. They hadundergone strong fracturing and were thus squeezed into different tectonic levels in the lowerpart of the Donghia Group complex and became deep-seated blocks in melanges of the Doughal subduction zone.