云南双沟蛇绿岩的特征和成因

Characteristics and Genesises of ShuanggouOphiolites, Yunnan Province, China

双沟蛇绿岩产于古特提斯小洋盆中（石炭－叠纪）。蛇绿岩的洋壳很薄，未见层序，岩石组合简单，仅由玄武岩、辉长－辉绿岩两个单元组成，是板块慢速扩张、岩浆供给不充分条件下形成的。辉绿岩和玄武岩存在Ｎ－ＭＯＲＢ和Ｅ－ＭＯＲＢ两种类型，它们的微量元素丰度和比值不同，指示来自不同的地幔源区。双沟洋壳岩石的εＮｄ（ｔ）变化较大（＋１３．５～＋７．９，ｔ＝３４０Ｍａ），（８７Ｓｒ／８６Ｓｒ）ｉ（０．７０３９～０．７０５４）和（２０８Ｐｈ／２０４Ｐｈ）ｉ（３７．４～３７．６）比值高，（２０６Ｐｈ／２０４Ｐｈ）ｉ比值低（１７．４～１７．８），表明存在ＤＵＰＡＬ异常的印记，与现今印度洋ＭＯＲＢ的同位素组成相似，而不同于北大西洋和太平洋ＭＯＲＢ的特征，可以用亏损地幔端元（ＤＭ）与第２类富集地幔端元（ＥＭ２）之间的混合来解释。

s: The Shuanggou ophiolite exposed in the Ailaoshan ocean basin is located between theYangtze Plate and the Lincang Plate, and mainly composed of three rock units: meta-peridotite, gabbro-diabaseand basalt. The ocean crust of the Shuanggou ophiolite is very thin and doesn't have complete sequence. Theassemblage of rocks is very simple, it only includes bsalts and gabbro--diabases. The contact relation betweenbasalts and gabbro--diabases are not found by field observation, it is probable that gabbro--diabases are layingover the upper mantle peridotite directly. The ENd (t) of Shuanggou ocean crust rocks (gabbro, diabases, basalt,quartz-diorite) is relatively high and varies in a wide range(from + 7. 9 to + 13. 5). (87Sr/86Sr), ratio is alsorather high (between 0. 7039 and 0. 7054, MORB is 0. 7023 ￣ 0. 7038). (206Pb/204Pb), ratio of the rocks isrelatively low (17. 4￣17. 8),but its(208Pb/204Pb). ratio is high (37. 4￣ 37. 6) ploting above the NHRL in thePh--Ph diagram. High 87Sr/86Sr and 208Pb/204Pb ratios show the exsistence of DUPAL anomaly and indicate thatShuanggou ophiolite was affected by the materials from lower mantle. Probably, the source of Shuanggouophiolite is mixed product of depleted mantle member(DMM) and the second kind of enriched mantle member(EM2). The Ph isotope data indicate that the DMM perhaps were contaminated by the isotope print fromancient continental lithosphere, but it's not the MORB of modern North Atlantic or East Pacific as generallythought. The light depleted lherzolite takes the most part of the rocks of Shuanggou and harzburgite occupies thesecond position. The rocks supply the evidence of partial melting and metasomatism of mantle. So,Shuanggouophiolite has distinctive characteristics: its rock a￣iation and geochemical characteristics are entirely differentfrom Troodos ophiolite. The latter has complete succession of strata, great thickness, strong depleted mantle,and its lavas show geochemical characteristics of island-arc settings. We can say that both Shuanggou andTroodos ophiolites were formed in small ocean basin, the Shuanggou ophiolite could be product of slowspreading, while Troodos ophiolite is related to island-arc spreading over subduction zone.