A Study of Recently Uplifted Upper Mantle and Lower Crustal Sections from the Red Sea: Implications for Rifting and the Opening of Oceans

本项目是研究在红海Zabargad岛暴露的上地幔橄榄岩剖面和下地壳麻粒岩相复合体，作为继续研究上地幔和下地壳在大陆裂谷向海洋裂谷演化过程中的演化的一部分。地壳杂岩含石榴石-麻粒岩相硅酸片麻岩、角闪化变质长辉长岩和变质辉长岩。硅质片麻岩可能是裂谷前的残余；下大陆地壳形成；在泛非周期期间高磷变质火成岩被解释为在裂谷之前和裂谷期间在地壳下包裹的变质基性岩浆。橄榄岩为同位素贫化尖晶石辉橄榄岩，由具有低87Sr/86Sr和143Nd/144Nd同位素特征的次大陆地幔交代流体部分水合成角闪孔橄榄岩。它们被认为是红海下地幔的一部分。从麻粒岩到角闪岩再到绿片岩相条件，地壳杂岩和橄榄岩均表现出连续的再平衡组合，表明它们是在深部构造水平上一起隆升的。pi将测试一个假设，即整个部分曾经非常接近泛非地壳的底部，最近迅速隆起并暴露出来，可能是沿着一条可能在扎巴尔加德暴露的主要分离断层，这是红海开放的结果。研究计划是扎巴尔加德橄榄岩和地壳岩石的岩石学、主要元素、微量元素和稀土元素、矿物化学以及Sr、Nd、Pb和可能的Os同位素。一个主要目标将是确定地壳岩石的初始年龄、再结晶年龄和隆升速率。Lamont-Doherety地质观测站的Rb-Sr、Sm-Nd和U-Pb全岩和矿物结果将与其他实验室的U-Pb锆石、40Ar/39Ar角闪洞、锆石、榍石和磷灰石裂变径迹年龄进行协调。目的是(a)确定扎巴尔加德杂岩隆升的时间、速率和机制，(b)确定过渡性裂谷中下大陆地壳的性质，(c)研究前大洋裂谷的底沉积过程，(d)阐明海底扩张之前大裂谷中地壳和地幔的岩石学和地球化学演化，(e)推断红海裂谷中是“主动”还是“被动”裂谷。该项目的预期结果将有助于我们了解大西洋型被动边缘的地壳演化。

This project is to study a section of upper mantle peridotites and a lower crustal granulite-facies complex exposed on the island of Zabargad, Red Sea, as part of a continuing study on the evolution of the upper mantle and lower crust as continental rifts evolve to oceanic rifts.The crustal complex contain garnet- granulite-facies silicic gneisses, and amphibolitized metagabbros and metapyroxenites. The silicic gneisses are probably remnants of the pre-rift ;lower continental crust formed ;during the Pan African cycle. The high-P meta-igneous rocks are interpreted as metamorphosed basic magmas that underplated the crust prior to and during rifting. the peridotites include isotopically depleted spinel lherzolites that were partially hydrated to amphibole peridotites by fluids with the isotopic signature (low 87Sr/86Sr and 143Nd/144Nd) of subcontinental mantle metasomatism. They are believed to have been part of the sub-Red Sea mantle. Both the crustal complex and the peridotites show successive re- equilibration assemblages from granulite to amphibolite to greenschist facies conditions suggesting they were uplifted together from deep tectonic levels. The PIs will test the hypothesis that the entire section was once very close to the base of the Pan African crust and was raised and exposed recently and rapidly, possibly along a major detachment fault that may be exposed on Zabargad, as a result of the opening of the Red Sea. Schedule for study are petrology, major, trace and Ree elements, mineral chemistry, and Sr, Nd, Pb and possibly Os isotopes of the Zabargad peridotites and crustal rocks. A major goal will be to determine initial ages, recrystallization ages and uplift rates of the crustal rocks. Rb-Sr, Sm-Nd and U-Pb whole-rock and mineral results from Lamont-Doherety Geological Observatory will be coordinated with U-Pb zircon, 40Ar/39Ar amphibole, and zircon, sphene and apatite fission track ages from other laboratories. The objectives are (a) to determine the time, rate and mechanism of uplift of the Zabargad complex, (b) define the nature of the lower continental crust in a transitional rift, (c) investigate the processes of underplating in a pre-oceanic rift, (d) clarify the petrological and geochemical evolution of the crust and mantle in a major rift prior to sea floor spreading, and (e) infer whether "active" or "passive" rifting prevailed in the Red Sea Rift. The results expected from this project will help us understand the crustal evolution of Atlantic-type passive margins.