[show abstract][hide abstract] ABSTRACT: Carbonatites are redefi ned using a mineralogical-genetic classifi cation and divided into two groups: primary carbonatites, and carbothermal residua. Attention is drawn to the fact that carbonatite is both a petrographic term applicable to a particular rock-type as well as a group name applied to a complex of related carbonate and silicate rocks in a magmatic or extrusive complex. Primary carbonatites, in terms of mineralogical-genetic classifi cations, rather than simple modal classifi cations, can be divided into a group of bona fi de magmatic carbonatites formed from diverse mantle-derived magmas, i.e., carbonatites associated with the melilitite, nephelinite, aillikite and kimberlite clans, with the latter best being termed calcite kimberlites. Each magma type and associated carbonatites are considered to be genetically distinct, and formed at different depths in the upper mantle by different degrees of partial melting. Carbonatites associated with the melilitite and nephelinite clans can have a multiplicity of origins, and may be formed by fractional melting, fractional crystallization or liquid immiscibility. Calcite kimberlites are small-volume late-forming differentiates that are not related to other carbonatites or their parental magmas. The origin and genetic relationships of the Oldoinyo Lengai natrocarbonatite cannot be unambiguously determined, although these rocks are regarded as a distinct variety of primary carbonatite. Carbonate-rich rocks associated with diverse potassic or sodic peralkaline saturated to undersaturated magmas derived predominantly from metasomatized lithospheric mantle, together with REE-carbonate-rich rocks of undetermined genesis, are best termed carbothermal residua rather than carbonatite. There can be mineralogical (or modal) convergence between these rocks and low-pressure REE-rich derivatives of bona fi de primary carbonatites. Carbonate-rich rocks formed by pneumatolytic reactions or anatectic melting of crustal rocks should not be considered to be carbonatites.
The Canadian Mineralogist 01/2049; 43:2049-2068. · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Barabazar granite, exposed at the northern margin of Singhbhum craton, Eastern India, occurs along the South Purulia Shear
Zone (SPSZ) and is emplaced into the Palaeoproterozoic metapelites and felsic volcanics of Singhbhum Group. Geochemical, petrographical
and geochronological studies on the Barabazar granite addressed in the work have wide implications on understanding the geodynamics
of SPSZ during Palaeoproterozoic to Mesoproterozoic. Geochemically, Barabazar granite displays limited range of major oxides,
alkali enrichment and highly fractionated features (SiO2 > 75%; Eu/Eu* = 0.16–0.33; enrichment of K, Rb, Th, U and Nb; depletion of Ba, Sr, P and Ti). It is predominantly peraluminous
(molar Al2O3/CaO+Na2O+K2O (A/CNK) =1.14–144) and contains abundant alkali feldspar, perthite, and minor plagioclase, biotite and accessory minerals.
Geochemical and petrological data indicates that it is A-type granite, which formed in ‘Within plate granite’ tectonic set
up. The Barabazar granite was emplaced at ca. 1771 Ma (Pb-Pb) in rift related environs and evolved by partial melting of stabilized
lower/middle crust (initial 87Sr/86Sr = 0.7302 ± 0.0066 and μ1 = 8.5 ± 0.5). Subsequently, the shear zone (SPSZ) developed during the closure of the riftogenic basin and was reactivated
during the Grenvillian orogeny (Ca. 900–1300 Ma), resulting in rehomogenisation of the strontium isotopes and thereby yielding
younger whole-rock Rb-Sr isotope age of c. 971 Ma for the Barabazar granite. Probably during this tectonic event, the Singhbhum
craton (Southern India Shield) would have finally juxtaposed with Northern Indian Shield along Central Indian Tectonic Zone
(CITZ) during the global Grenvillian orogeny.
KeywordsA-type Barabazar granite–Geochemistry–Geochronology–Grenvillian orogeny–Singbhhum craton–SPSZ–West Bengal
Journal of the Geological Society of India 05/2012; 77(6):527-538. · 0.57 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Hufang composite pluton of the Qingliu–Mingxi area of western Fujian is located in the eastern part of the South China Block (SCB). The pluton consists of three granitic intrusions: the Weipu (WP) alkali-feldspar-granite, the Shaxi (SX) biotite-granite, and the Qinxi (QX) alkali-feldspar-granite. Zircon U–Pb ages obtained for the three intrusions are 434 ± 3, 409 ± 3, and 224 ± 2 Ma, respectively, indicating that the Hufang composite pluton is made up of intrusions of both the Caledonian and Indosinian ages. All three intrusive bodies are weakly peraluminous to metaluminous and fall in the high-K calc-alkaline and shoshonite series. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and display moderate negative Eu anomalies and roughly flat patterns of heavy rare earth elements (HREEs). Integrated geological and geochemical data suggest that all three granitic intrusions are I-type and were emplaced in post-collisional extensional settings. All of the analysed rocks show homogeneous Nd isotopic compositions, with ϵNd(t) values ranging from −0.25 to +1.32 for the Caledonian SX biotite-granite and from −1.01 to −0.82 for the Indosinian QX alkali-feldspar-granite. However, they display highly variable zircon Hf isotopic compositions, with ϵHf(t) values of −5.9 to +3.8 and −2.4 to +10.2 for the Caledonian WP and SX granites, respectively, and −12.2 to −5.0 for the Indosinian QX granite. These Nd–Hf isotopic characteristics are best explained by binary mixing between juvenile, mantle-derived magma and the evolved crustal components. We suggest that crust–mantle interactions played an important role in the SCB generation of the Caledonian and Indosinian granitoids.
International Geology Review 01/2012; 54(1):15-32. · 3.36 Impact Factor
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