Mineralogy and geochemistry of microgranular enclaves in Palaeoproterozoic Malanjkhand granitoids, central India: evidence of magma mixing, mingling, and chemical equilibration
Mineralogy and geochemistry of microgranular enclaves in Palaeoproterozoic Malanjkhand granitoids, central India: evidence of magma mixing, mingling, and chemical equilibration
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DOI：
10.1007/s00410-006-0122-3
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发表时间：
2006-08
影响因子：
3.5
通讯作者：
Santosh Kumar;V. Rino
中科院分区：
文献类型：
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作者：
Santosh Kumar;V. Rino
印度中部前寒武纪地盾马兰杰坎德地区古元古代（约2,480 Ma）长英质岩浆活动，被称为马兰杰坎德花岗岩类（MG），包括围岩包体和中-暗色细粒斑状微粒砂岩（ME）。ME的形状是球形、椭圆形、盘形、细长形和透镜状，大小从几厘米到约2米不等。ME与主机MG的接触通常是尖锐的，细齿的，偶尔扩散，我们归因于ME球内较冷的主机MG的过冷和解聚。ME和MG均为半自形结构，具有相同的矿物组合Hbl-Bt-Kfs-Pl-Qtz，但模态比例不同。矿物成分的变化、磷灰石针状体、拉长的黑云母、再吸收的斜长石、单眼石英和其他镁铁质-长英质捕虏晶的存在强烈反对ME的残留和同源起源。（An 3-An 29），闪石（Mg/Mg+ Fe ~（2+）=0.55-0.69），黑云母（Mg/Mg+ Fe ~（2+）=0.46-0.60），表明镁铁质-长英质岩浆相互作用过程中存在部分或完全的平衡。角闪石中的铝估计MG岩体的侵位压力为3.4 ± 0.5千巴，因此岩浆混合作用和混合作用一定发生在这个水平或更低的水平。黑云母中ME和MG的替代在很大程度上表明了俯冲相关的长英质熔体的钙碱性金属（I型）性质。大多数主要和微量元素对SiO2产生近线性变化趋势的ME和MG，可能产生的混合镁铁质和长英质岩浆在不同的比例。然而，ME-MG对的痕量包括稀土元素图案显示部分到完全平衡，最有可能受不同程度的元素扩散的支配。现有的证据支持ME起源的模型，即同时代的镁铁质（包体）和长英质（MG）岩浆产生的混合（ME）岩浆层，注入到冷却器，部分结晶MG，分散，混合，和过冷的ME球在对流动力岩浆房。
Palaeoproterozoic (ca2,480 Ma) felsic magmatism of Malanjkhand region of central Indian Precambrian shield, referred to as Malanjkhand granitoids (MG), containxenolithsof country rocks and mesocratic to melanocratic, fine-grained porphyriticmicrogranularenclaves (ME). The shape of ME is spheroidal, ellipsoidal, discoidal, elongated, and lenticular, varying in size from a few centimeters to about 2 m across. The contact of ME with the host MG is commonly sharp, crenulate, and occasionally diffuse, which we attribute to the undercooling and disaggregation of ME globules within the cooler host MG. The ME as well as MG show hypidiomorphic texture with common mineral Hbl-Bt-Kfs-Pl-Qtz assemblage, but differ in modal proportions. The variation in minerals' composition, presence of apatite needles, elongated biotites, resorbed plagiclase, ocellar quartz, and other mafic–felsic xenocrysts strongly oppose the restite and cognate origins of ME. Compositions of plagioclases (An3–An29), amphiboles (Mg/Mg+Fe2+=0.55–0.69), and biotites (Mg/Mg+Fe2+=0.46–0.60) of ME are slightly distinct or similar to those of MG, which suggest partial to complete equilibration during mafic–felsic magma interactions. Al-in-amphibole estimates the MG pluton emplacement atca3.4 ± 0.5 kbar, and therefore, magma mixing and mingling must have occurred at or below this level. Thesubstitution in biotites of ME and MG largely suggests subduction-related, calc–alkaline metaluminous (I-type) nature of felsic melts. Most major and trace elements against SiO2produce near linear variation trends for ME and MG, probably generated by the mixing of mafic and felsic magmas in various proportions. Trace including rare earth elements patterns of ME–MG pairs, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. The available evidence supports the model of ME origin that coeval mafic (enclave) and felsic (MG) magmas produced a hybrid (ME) magma layer, which injected into cooler, partly crystalline MG, and dispersed, mingled, and undercooled as ME globules in a convectively dynamic magma chamber.