Geochemistry of Tuscan Archipelago Granitoids, Central Italy: The Role of Hybridization Processes in Their Genesis
Geochemistry of Tuscan Archipelago Granitoids, Central Italy: The Role of Hybridization Processes in Their Genesis
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DOI：
10.1086/629570
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发表时间：
1992-01
期刊：
影响因子：
--
通讯作者：
G. Poli
中科院分区：
文献类型：
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作者：
G. Poli
本文报道了位于意大利托斯卡纳群岛的厄尔巴岛、吉里奥岛和蒙特-基督山的选定样品的主要、痕量和稀土元素数据。根据野外和岩石学的限制，所有岩石可划分为5个相，组成范围从花岗闪长岩到碱花岗岩，以二长花岗岩为主。托斯卡纳群岛花岗岩类显然是与碰撞有关的岩石，它们横跨查普尔和怀特的I和S类型。然而，矿物学和化学特征表明，基性较强的岩石大致为I型，而高硅质岩石为S型。通过主矿物相和副矿物相的分离将这些岩浆联系起来的成因模型与主、微量和稀土元素丰富的数据不一致。地质、岩石学和地球化学的限制表明，在岩石中发现的微颗粒基性包裹体是基性高温岩浆团，在较冷、较硅的宿主中冷却，但简单的两端混合过程无法解释这些岩体的地球化学特征，无论是作为一个群体还是单个。一个两阶段的岩石成因模型解释了这些侵入岩所显示的所有地质、岩石学和地球化学特征：第一阶段是一个混合加晶体分馏（MFC）过程，涉及地壳过铝岩浆和可能起源于地壳下的基性岩浆；第二阶段是同一地壳过铝岩浆和基性岩浆衍生的不同演化岩浆之间的简单混合过程。第二阶段可能会因为可能的叠加过程而变得复杂。酸性端元可能为吉里奥岩体中的白岩相，而基性端元则根据年龄和地球化学特征与卡普拉亚岩相似。该模型还能够解释S型和I型花岗岩特征的二分性。微量元素数据表明，斜长石、钾长石、石英、独居石和锆石的结晶分馏过程是斜长石形成的。
This paper reports major, trace, and rare-earth-element data for selected samples from the Elba, Giglio, and Monte-cristo plutons, which lie in the Tuscan Archipelago (Italy). On the basis of field and petrologic constraints, all the rocks can be divided into five facies and range in composition from granodioritic to alkali-granitic, with a strong predominance of monzogranites. Tuscan Archipelago granitoids are clearly collision-related rocks, and they straddle the I and S types of Chappell and White. However, mineralogical and chemical characteristics indicate that the more basic rocks are broadly I type, while the high-silica rocks are S type. Genetic models that relate these magmas through the segregation of their major and accessory mineral phases are inconsistent with major, trace, and REE element abundant data. Geological, petrographical, and geochemical constraints show that the microgranular mafic enclaves found within the rocks are blobs of mafic, high-temperature magma chilled within a cooler, more silicic host, but a simple two-end-member mixing process fails to explain the geochemical characteristics of the plutons, either as a group or singly. A two-stage petrogenetic model explains all the geological, petrographical, and geochemical features shown by these intrusive rocks: first, a Mixing plus Crystal Fractionation (MFC) process involving a crustal peralumi-nous magma and a basic magma of probable subcrustal origin, and second, a simple mixing process between the same crustal peraluminous magma and different evolved magmas derived from the basic magma during the first stage. The second stage can be complicated by possible superimposed processes. The acid end-member could be the Leucocratic Facies found in the Giglio pluton, while the basic end-member should be similar to the Capraia rocks on the basis of age and geochemical characteristics. The model proposed is also able to explain the dichotomy between S and I granite characteristics. Trace element data support aplites genesis by a crystal fractionation process starting from the most evolved monzogranites and crystallizing an assemblage consisting of plagioclase, K-feldspar, quartz, monazite, and zircon.