Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift
Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift
复制标题
里奥格兰德裂谷中部地幔捕虏体中岩石圈去除的证据
DOI：
10.1016/j.epsl.2012.08.034
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发表时间：
2012
影响因子：
5.3
通讯作者：
J. C. Lassiter
中科院分区：
文献类型：
--
作者：
B. Byerly;J. C. Lassiter
中格兰德河裂谷（RGR）下的地震层析成像显示，在地幔中有一个低P波和S波速度带，向上延伸到莫霍面底部。该低速区域已由（Gao et al.，2004年）是对流移动的一部分，一旦> 100公里厚的元古代岩石圈的结果。中央RGR的伸展量被认为是低的（约25%），因此不能解释地震层析成像所建议的岩石圈减薄量。我们测量了整个岩石和矿物的主要元素，微量元素和同位素组成的尖晶石橄榄岩捕虏体爆发沿着的裂谷（象丘）和东部边缘的科罗拉多高原（塞罗查托），以确定其深度的起源和地幔源区，并测试拆沉假说。如果没有发生岩石圈的移除和低的P和S波速度，而不是在岩石圈中的水化或熔体渗透的结果，则在裂谷轴上喷发的捕虏体应该具有类似于元古代次大陆岩石圈地幔（SCLM）的地球化学成分。在位于科罗拉多高原边缘的CerroChato，捕虏体产于60 km深处，具有类似于元古宙陆下岩石圈地幔的地球化学特征（如难熔主元素组成、轻稀土富集、Sr和Nd同位素富集、非放射成因的Os同位素）。在象丘，沿着中央裂谷轴，存在两组不同的捕虏体。象丘捕虏体中的轻稀土元素亏损，主量元素含量丰富。此外，这些捕虏体的同位素特征与DMM的范围相似（例如，87 Sr/86 Sr范围为0.7018至0.7023，ε N范围为7至21，187 Os/188 Os范围为0.122至0.130）。我们认为这组捕虏体来自软流圈地幔。象丘的一组丰度较低的捕虏体富含轻稀土元素，具有难熔主要元素成分、富集的Sr、Nd和Pb同位素以及非放射成因的Os同位素。这些都是元古代SCLM的特征。象丘中的两组捕虏体均来自45 km深度。我们解释套件捕虏体在象丘采样什么是最近的基地元古代SCLM。我们的结论是，地幔岩石圈的一部分已被删除，使现代对流地幔（DMM）的基础上，预先存在的SCLM的侵位。
Seismic tomography beneath the Central Rio Grande Rift (RGR) at ∼34°N shows a low P and S wave velocity zone in the mantle that extends up the base of the Moho. This low-velocity region has been interpreted by (Gao et al., 2004) to be the result of convective removal of a portion of the once >100km thick Proterozoic lithosphere. The amount of extension in the central RGR is thought to be low (∼25%) and thus cannot account for the amount of lithosphere thinning suggested by seismic tomography. We measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis. If lithosphere removal has not occurred and the low P and S wave velocities are instead the result of hydration or melt infiltration in the lithosphere, then xenoliths erupted on the rift axis should have geochemical compositions similar to Proterozoic sub-continental lithospheric mantle (SCLM). At Cerro Chato, on the margin of the Colorado Plateau, xenoliths were derived from ∼60km depth and have geochemical signatures similar to Proterozoic sub-continental lithospheric mantle (e.g. refractory major element compositions, LREE-enrichment, enriched Sr and Nd isotopes, unradiogenic Os isotopes). At Elephant Butte, along the central rift axis, two distinct groups of xenoliths are present. The majority of xenoliths from Elephant Butte are LREE-depleted and have fertile major element compositions. Additionally, these xenoliths have isotopic signatures similar to the range for DMM (e.g.87Sr/86Sr ranging from 0.7018 to 0.7023, εNdranging from 7 to 21, and187Os/188Os ranging from 0.122 to 0.130). We interpret this group of xenoliths to be derived from asthenospheric mantle. A less-abundant group of xenoliths at Elephant Butte are LREE enriched, have refractory major element compositions, enriched Sr, Nd, and Pb isotopes, and unradiogenic Os isotopes. These are characteristic of Proterozoic SCLM. Both groups of xenoliths from Elephant Butte are derived from ∼45km depth. We interpret the suite of xenoliths at Elephant Butte to have sampled what was recently the base of the Proterozoic SCLM. We conclude that a portion of the mantle lithosphere has been removed which allowed modern convecting mantle (DMM) to be emplaced at the base of the pre-existing SCLM.