The Mannapra syenite, Central Kerala, India: Geochemistry, petrogenesis and bearing on anorogenic magmatism

Abstract

Occurrence of a syenite body near Mannapra, Trichur district, Kerala, is reported. The syenite, emplaced within charnockitic country rocks along the Idamalayar faultlineament, is exposed over an area of 8 km 2. The rock, classified as quartz alkali feldspar syenite based on Q-A-P proportions, has alkali feldspar as the dominant mineral constituent which shows variation in perthitic texture from r to micro-and patch-perthites. Both ortho-and clino-pyroxenes are present, with the former showing blebs of plagioclase suggesting Al-unmixing. Clinopyroxene shows subsolidus reequilibration with greenish pleochroic grain margins and local conversion to alkali amphibole. The petrochemical characters indicate that the syenite crystallized from a partial melt which equilibrated from Krich, Rb-depleted source in the upper mantle, in response to crustal distension and mantle degassing prior to the rifting of the continent. Reaction between early formed minerals and the late peralkaline liquid towards the residual phase resulted in subsolidus reequilibration textures. The syenite is envisaged to be yet another example of the manifestation ofanorogenic magmatism in this part of the Indian shield.

Radioelement geochemistry of alkali granites of the Kerala region, south-west India

“…The K-rich nature is a common characteristic of the alkaline plutons in the Kerala region, which, together with the trace element characters suggest magma derivation by partial melting of a K-enriched, Rb-depleted deep crustal or upper mantle source (cf. Nair and Santosh 1984;Santosh and Thara 1985). There is a consistent homogeneity in the distribution of K in individual plutons yielding low standard deviation values (0"49-0.90).…”

“…U is released from the mantle not only into the magma but also directly into the volatile phases. Petrochemical and tectonic studies of the alkaline plutons suggest their derivation from partial melts generated in the mantle (Santosh and Thara 1985). An intrinsic association of volatiles with the petrogenesis of these plutons is also suggested (Nair and Santosh 1984).…”

“…The region comprises largely Precambrian crystallines including charnockites, khondalites and migmatitic gneisses. Recent studies reveal the occurrences of a number of granite and syenite intrusives representing a Late Precambrian-Early Paleozoic magmatic regime (Santosh and Nair 1983a;Santosh and Thara 1985;Nair and Santosh 1984). The spatial relationship of the intrusives with regional fault-lineaments suggests a taphrogenic association (Santosh and Nair 1983b;Nair et al 1983).…”

Geochemistry and petrogenesis of syenites and associated rocks of the Elagiri complex, Southern Granulite Terrane, India

“…Most of the well described plutons correspond to silica undersaturated complexes such as those of Koraput (Bose, 1970), Sivamalai (Bose, 1971), Kunavaram , and Elchuru (Nag, 1983;Bose and Nag, 1980). However, some silica-oversaturated alkaline complexes have also been described from the SGT including those of Mannapra (Santosh and Thara, 1985), Peralimala (Nair and Santosh, 1984), Munnar (Nair et al, 1983), and Puttetti (Nair and Santosh, 1985). The Elagiri complex in the Tamil Nadu State of southern India (12°31 0 N, 78°35 0 E) has been described in the literature as a lineament-controlled oversaturated alkaline pluton (Grady, 1971;Sugavanam et al, 1976;Ratnakar and Leelanandam, 1989;Rajesh and Santosh, 1996;Santosh et al, 2009).…”

Significance of Mineral Chemistry of Syenites and Associated Rocks of Elagiri Complex, Southern Granulite Terrane of the Indian Shield

“…However, petrological details of some saturated-oversaturated alkaline complexes of southern granulite terrane are also available in literature namely Mannapra (Santosh and Thara, 1985), Peralimala (Nair and Santosh, 1984), Munnar (Nair et al 1983) and…”