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

该项目旨在研究出露在红海扎巴尔加德岛上的一段上地幔橄榄岩和下地壳麻粒岩相杂岩，作为大陆裂谷演化为大洋裂谷时上地幔和下地壳演化的持续研究的一部分。地壳杂岩包括石榴麻粒岩相硅质片麻岩、角闪化变质辉长岩和变质辉石岩。硅质片麻岩可能是前裂谷的残留物；形成于泛非旋回的下陆壳。高磷变质火成岩被解释为在裂谷之前和裂谷期间形成地壳底板的变质基性岩浆。橄榄岩包括同位素亏损的尖晶石二辉橄榄岩，这些尖晶石二辉橄榄岩被具有陆下地幔交代作用的同位素特征(低87Sr/86Sr和143Nd/144Nd)的流体部分水化成角闪石橄榄岩。它们被认为是次红海地幔的一部分。地壳杂岩和橄榄岩表现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共同抬升的。PI将检验这一假设，即整个区段曾经非常接近泛非地壳的底部，并在最近迅速隆起和暴露，可能是沿着扎巴尔加德的一个主要拆离断层暴露的，这可能是红海开放的结果。研究范围包括岩石学、主量元素、痕量和稀土元素、矿物化学以及扎巴尔加德橄榄岩和地壳岩石的锶、钕、铅和可能的Os同位素。一个主要目标将是确定地壳岩石的初始年龄、重结晶年龄和抬升速率。Lamont-Doherty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锆石、40Ar/39Ar角闪石和锆石、辉石和磷灰石裂变径迹年龄相协调。其目标是：(A)确定扎巴尔加德杂岩的抬升时间、速度和机制；(B)确定过渡性裂谷中下大陆地壳的性质；(C)调查大洋裂谷中的底侵作用；(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)推断红海裂谷中普遍存在的是“主动”裂谷作用还是“被动”裂谷作用。这一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.