Mineralogy of ultramafic cumulates and tectonites from eastern Papua
Mineralogy of ultramafic cumulates and tectonites from eastern Papua
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巴布亚东部超镁铁质堆积物和构造岩的矿物学
DOI：
10.1016/0012-821x(73)90210-0
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发表时间：
1973
影响因子：
5.3
通讯作者：
H. Davies
中科院分区：
文献类型：
--
作者：
R. N. England;H. Davies
巴布亚超镁铁质带的超镁铁质岩石根据其结构可分为两类：(1)具有表明可能是玄武岩岩浆结晶沉降形成的结构的岩石（超镁铁质堆积）；(2)具有固态再结晶结构的岩石（超镁铁质构造岩）。认为超镁铁质堆积物是由玄武岩侵入体的结晶沉降形成的，这些玄武岩侵入体现在以超镁铁质带的上覆辉长岩带为代表，超镁铁质构造岩是先前存在的上地幔，为玄武岩侵入体形成了底板。用电子探针对两种类型的橄榄石、正辉石、斜辉石和铬尖晶石进行了分析。10个超镁铁质堆积中橄榄石和正辉石的组成在fo78.3 ~ 89.6和en81.2 ~ 90.5范围内，而13个超镁铁质构造岩样品中相同矿物的组成在fo91.6 ~ 93.6和en92.1 ~ 93.4范围内。超镁铁质堆积物的矿物组成范围广泛，与分选岩浆结晶沉降的成因相吻合，但超镁铁质构造岩中矿物组成范围很小，具有均匀的高镁质特征，体积大，地理分布广，强烈表明这些岩石不是再结晶堆积物。这种特征可以从原始地幔物质部分熔融过程中形成的难熔残留物的岩石中预料到。然而，超镁质构造岩中许多正辉石的Ca和Al含量低于与玄武岩岩浆平衡的残余。
Ultramafic rocks in the Papuan Ultramafic Belt can be divided into two categories on the basis of their fabric: (1) rocks with textures which indicate origin by crystal settling from presumably basaltic magma (ultramafic cumulates); and (2) rocks with textures which suggest recrystallization in the solid state (ultramafic tectonites). It is thought that the ultramafic cumulates have formed by crystal settling from the basaltic intrusions which are now represented by the overlying gabbro zone of the Ultramafic Belt, and that the ultramafic tectonites are pre-existing upper mantle and formed a floor for the basaltic intrusions [6].Olivine, orthopyroxene, clinopyroxene and chrome spinel from both types were analysed by electron probe. Olivine and orthopyroxene from ten ultramafic cumulates have compositions in the ranges Fo78.3–89.6and En81.2–90.5, whereas the same minerals in thirteen samples of ultramafic tectonite have compositions in the ranges Fo91.6–93.6and En92.1–93.4. The wide range of mineral compositions in the ultramafic cumulates is compatible with an origin by crystal settling from a fractionating magma, but the very small compositional range and uniformly highly magnesian character of minerals from the ultramafic tectonites, together with their large volume and wide geographic distribution, strongly suggest that these rocks are not recrystallized cumulates. Such characteristics would be expected from rocks which formed as a refractory residue during partial melting of primitive mantle material. However, the Ca and Al contents of many orthopyroxenes in the ultramafic tectonites are lower than would be expected for a residue in equilibrium with basaltic magma.