An overview on geochemistry of Proterozoic massif-type anorthosites and associated rocks

Abstract

A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite, anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na 2 O + K 2 O, Al 2 O 3 , SiO 2 , Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ 18 O (5.7-7.5), initial 87 Sr/ 86 Sr (0.7034-0.7066) and ε Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease in plagioclase content, An content of plagioclase and X Mg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from anorthositemafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically, the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ 18 O values (6.8-10.8 ) and initial 87 Sr/ 86 Sr (0.7067-0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors.

Petrology and geochemistry of the Batchingou anorthositic suite rocks (Bana volcano‐plutonic complex, Cameroon Volcanic Line): Inference on their origin and relation with host granites

“…The divider suggested by Irvine and Baragar (1971) to separate tholeiitic (above) from calc alkalic (below) suites is shown as red line for reference only. Field's boundaries are drawn based on diagrams compiled by Maji, Patra, and Ghosh (2010)…”

“…Rb (ppm) vs. Rb/Sr × 1,000 of anorthosite, mafic‐leucomafic rocks, Fe‐rich dioritic rocks and K‐rich felsic suite of rocks from massif anorthosite complexes and weight (%) FeO total + MnO + MgO + TiO 2 vs. Sr (ppm) of anorthosite, mafic‐leucomafic rocks, Fe‐rich dioritic rocks, and K‐rich felsic suite of rocks from massif anorthosite complexes. Field's boundaries are drawn based on diagrams compiled by Maji et al (2010)…”

Geological, petrological, and geodynamical characteristics of the Karacaali Magmatic Complex (Kırıkkale) in the Central Anatolian Crystalline Complex, Turkey

“…There are some suggestions related to the genesis and association of anorthositic rocks with the granitic intrusions: i) fractional crystallization of melts of upper mantle or deep crustal origin (Duchesne and Demaiffe, 1978); ii) differentiation of basaltic magma and accumulation of buoyant plagioclase at the top of deepseated magma chambers (i.e. at the crust-mantle interface or in the lower crust), then followed by diapiric uprising of low-density plagioclase mushes toward midcrustal level in the anorthositic composition due to gravitational instability (Czamanske and Bohlen, 1990;Longhi et al, 1993;Mitchell et al, 1996;Namur et al, 2011); iii) partial melting of the lower crust resulting in intrusion of both anorthositic and granitic magma (Fazlnia et al, 2007); iv) fractionation of mantle-derived melts resembling high-Al gabbro of varied composition (Maji and Ghosh, 2010). Of these, presumably, anorthositic magma in the hybrid rocks of KMC formed by differentiation of basaltic magma and accumulation of buoyant plagioclase at the top of deepseated magma chambers (i.e.…”

Petrological and geochemical studies of ultramafic–mafic rocks from the North Puruliya Shear Zone (eastern India)

“…Group 1 with lower colour index (<90) represents mafic rocks (vide figures 2a, b, 3, 4a, b) and Group 2 with higher colour index (>90) represents ultramafic rocks (vide figures 2a, 3, 4a, b). Important occurrences of mafic rocks from other areas (not along North Puruliya Shear Zone) in CGC are Saltora anorthosite-gabbro suite (Maji et al 2010); dolerite and norite from NW of Jhalida (Sengupta 1958;Singh 1959) and gabbroic rocks from Belamu-Jaipur area (Baidya and Chakravarty 1988). All these mafic-ultramafic rocks are deformed and metamorphosed.…”