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

本项目是研究红海扎巴加德岛上出露的一段上地幔橄榄岩和一个下地壳麻粒岩相杂岩，作为继续研究大陆裂谷向大洋裂谷演化过程中上地幔和下地壳演化的一部分，该地壳杂岩包括石榴麻粒岩相长角闪岩、角闪岩化变辉长岩和变辉石岩。这些片麻岩可能是前裂谷的残余物;下陆壳形成;在泛非周期。高磷变质火成岩被解释为变质的基性岩浆底侵地壳之前和期间裂谷。橄榄岩包括同位素贫化的尖晶石二辉橄榄岩，其被具有次大陆地幔交代作用的同位素特征（低87 Sr/86 Sr和143 Nd/144 Nd）的流体部分水合为角闪石橄榄岩。他们被认为是红海地幔的一部分。地壳杂岩和橄榄岩都显示出从麻粒岩到角闪岩再到绿片岩相的连续再平衡组合，表明它们是从深部构造水平一起抬升的。初步研究报告将检验这样一种假设，即整个剖面曾经非常接近泛非地壳的底部，最近迅速上升和暴露，可能是由于红海的开放，沿着可能在扎巴加德暴露的一个主要拆离断层。研究计划包括岩石学、主量元素、微量元素和稀土元素、矿物化学、以及扎巴尔加德橄榄岩和地壳岩石的Sr、Nd、Pb和可能的Os同位素。一个主要目标是确定地壳岩石的初始年龄、重结晶年龄和隆升速率。Lamont-Doherty地质观测站的Rb-Sr、Sm-Nd和U-Pb全岩矿物结果将与其他实验室的U-Pb锆石、40 Ar/39 Ar角闪石以及锆石、辉石和磷灰石裂变径迹年龄进行协调。其目的是：（a）确定扎巴尔加德杂岩隆升的时间、速率和机制;（B）确定过渡裂谷中下陆壳的性质;（c）研究前大洋裂谷中底侵作用的过程;（d）澄清海底扩张之前主要裂谷中地壳和地幔的岩石学和地球化学演化;以及（e）推断红海裂谷中是否存在“主动”或“被动”裂谷作用。该项目的研究成果将有助于我们了解南极型被动大陆边缘的地壳演化。

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.