Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite
Along and across arc geochemical variations in NW Central America: Evidence for involvement of lithospheric pyroxenite
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DOI：
10.1016/j.gca.2012.01.035
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发表时间：
2012-05
影响因子：
5
通讯作者：
K. Heydolph;K. Hoernle;F. Hauff;P. Bogaard;M. Portnyagin;I. Bindeman;D. Garbe‐Schönberg
中科院分区：
文献类型：
--
作者：
K. Heydolph;K. Hoernle;F. Hauff;P. Bogaard;M. Portnyagin;I. Bindeman;D. Garbe‐Schönberg
中美洲火山弧（CAVA）在过去几年中一直是深入研究的主题，导致了各种不同的模型CAVA熔岩的起源与不同的源成分。我们提出了一个新的模型，西北中美洲火山弧的基础上，一个全面的新的地球化学数据集（主要和微量元素和Sr-Nd-Pb-Hf-O同位素比值）的基性火山前（VF），背后的火山前（BVF）和弧后（BA）熔岩和火山灰样品从西北尼加拉瓜，洪都拉斯，萨尔瓦多萨尔瓦多和危地马拉。此外，我们目前的数据俯冲科科斯板块沉积物（从DSDP腿67站点495和499）和火成岩洋壳（从DSDP腿67站点495），危地马拉（Chortis块）花岗岩和变质大陆基底。我们观察到微量元素和同位素组成的系统变化都沿着和整个弧。数据需要至少三个不同的端元在中美洲西北部的火山作用。(1)西北尼加拉瓜VF熔岩需要一个端员非常高的Ba/（La，Th）和U/Th，相对放射性的Sr，Nd和Hf，但非放射性的铅和低δ 18 O，反映了主要蛇纹岩衍生的流体/含水熔体通量从俯冲板片到一个耗尽的N-MORB型地幔楔。(2)Guatemala VF和BVF基性熔岩要求具有低Ba/（La，Th）、U/Th、高δ 18 O和放射成因Sr和Pb同位素比值，但非放射成因Nd和Hf同位素比值的富集端元。Hf与Nd和Pb同位素组成的相关性与该端员是俯冲沉积物不一致。来自危地马拉奇基穆拉深成杂岩的花岗岩样品具有作为该端元的适当同位素组成，但解释同位素数据所需的大量同化与火山岩的玄武岩组成不一致。此外，Nd与Hf的混合回归以及Sr和O同位素图没有通过数据。因此，我们建议，这端员可以代表辉石在岩石圈（地幔和可能的下地壳），来自母岩浆的深成岩。(3)洪都拉斯和加勒比海的BA熔岩定义了一个同位素亏损的端员（与非放射性Sr，但放射性Nd，Hf和Pb同位素比值），具有OIB样的主要和微量元素组成（如低Ba/（La，Th）和U/Th，高La/Yb）。这端元可能是来自年轻的，回收的洋壳在软流圈上涌弧后熔融。这三种端元类型的岩浆之间的混合可以解释所观察到的系统的地球化学变化沿着和横跨西北中美洲弧。
The Central American Volcanic Arc (CAVA) has been the subject of intensive research over the past few years, leading to a variety of distinct models for the origin of CAVA lavas with various source components. We present a new model for the NW Central American Volcanic Arc based on a comprehensive new geochemical data set (major and trace element and Sr–Nd–Pb–Hf–O isotope ratios) of mafic volcanic front (VF), behind the volcanic front (BVF) and back-arc (BA) lava and tephra samples from NW Nicaragua, Honduras, El Salvador and Guatemala. Additionally we present data on subducting Cocos Plate sediments (from DSDP Leg 67 Sites 495 and 499) and igneous oceanic crust (from DSDP Leg 67 Site 495), and Guatemalan (Chortis Block) granitic and metamorphic continental basement. We observe systematic variations in trace element and isotopic compositions both along and across the arc. The data require at least three different endmembers for the volcanism in NW Central America. (1) The NW Nicaragua VF lavas require an endmember with very high Ba/(La, Th) and U/Th, relatively radiogenic Sr, Nd and Hf but unradiogenic Pb and low δ18O, reflecting a largely serpentinite-derived fluid/hydrous melt flux from the subducting slab into a depleted N-MORB type of mantle wedge. (2) The Guatemala VF and BVF mafic lavas require an enriched endmember with low Ba/(La, Th), U/Th, high δ18O and radiogenic Sr and Pb but unradiogenic Nd and Hf isotope ratios. Correlations of Hf with both Nd and Pb isotopic compositions are not consistent with this endmember being subducted sediments. Granitic samples from the Chiquimula Plutonic Complex in Guatemala have the appropriate isotopic composition to serve as this endmember, but the large amounts of assimilation required to explain the isotope data are not consistent with the basaltic compositions of the volcanic rocks. In addition, mixing regressions on Nd vs. Hf and the Sr and O isotope plots do not go through the data. Therefore, we propose that this endmember could represent pyroxenites in the lithosphere (mantle and possibly lower crust), derived from parental magmas for the plutonic rocks. (3) The Honduras and Caribbean BA lavas define an isotopically depleted endmember (with unradiogenic Sr but radiogenic Nd, Hf and Pb isotope ratios), having OIB-like major and trace element compositions (e.g. low Ba/(La, Th) and U/Th, high La/Yb). This endmember is possibly derived from melting of young, recycled oceanic crust in the asthenosphere upwelling in the back-arc. Mixing between these three endmember types of magmas can explain the observed systematic geochemical variations along and across the NW Central American Arc.