Presence Of Mafic Rocks Research Articles

Nature of the Moho in the mid-eastern part of the Chotanagpur Plateau, India, from a receiver function perspective

The present study aims to find out the crustal thickness, VP/VS ratio, and Poisson’s ratio beneath the Indian School of Mines (ISM) broadband seismic station using receiver function analysis. The station is located at latitude 23.82° N and longitude 86.44° E in the mid-eastern part of the Chotanagpur Plateau under the Eastern Indian Shield region. A total of 300 teleseismic events (M ≥ 6.0), recorded at the ISM station during 2007–2012, were compiled, and 97 good-quality events selected for the analysis. Receiver function analysis and H–k stacking method are used to determine the crustal thickness (H) and VP/VS ratio. The depth of Mohorovicic (Moho) discontinuity below the station is estimated to be ∼42.2 km with an average VP/VS ratio of ∼1.92. We also find the presence of Conrad discontinuity at a depth of ∼20 km. The computed average Poisson’s ratio of ∼0.31 for the whole crust beneath the ISM broadband station is relatively higher and might be indicating the presence of mafic rocks or partial melt near the Moho. Similar observations were also reported from different segments of the Son-Narmada-Tapi mega-shear zone, a westward extended part of the present study area.

Evidence of Protolith Contamination in the Generation of an Anatectic Complex: Peña Negra, Central Spain

The Pena Negra complex, in central Spain, provides a superb opportunity for the study of Variscan partial melt- ing processes. The heat source for the anatexis has been controversial. The presence of mafic rocks in the vicinity has given rise to the suggestion that mafic magmas of mantle origin provided the requisite heat for the generation of the anatectic granites. Experimental petrology can be used to support this concept, but the volumes of mafic magma necessary for the anatexis appear improbably large. Geophysical models for Central Iberia do not support the presence of such large basic complexes at depth. Furthermore radiometric dating of the scarce mafic outcrops indicates that they are too old to have been involved in the granitic magma genesis. The alternative interpretation is that melting of the protolith was pro- moted by the radioactive decay of K, U and Th. Anatexis was enhanced by the existence of a fertile protolith and by the presence of shear structures that permitted volatile fluxing. The situation is believed to have been analogous to that in other European Variscan terrains.

Geology and geochemistry of arenite–quartzwacke from the Late Archaean Sandur schist belt—implications for provenance and accretion processes

Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO 2, 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al 2O 3 and TiO 2 in both these rock types. The ACNK modeling of arenites and quartzwackes show evidence for addition of K 2O during later metasomatic alteration. In the ACNKFM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt.

MAFIC ROCKS FROM THE SEDIMENTARY BRECCIAS ASSOCIATED TO THE BALAGNE OPHIOLITIC NAPPE (NORTHERN CORSICA): GEOCHEMICAL FEATURES AND GEOLOGICAL IMPLICATIONS

In the Balagne area (Northern Corsica), two main coarse-grained deposits characterized by the presence of mafic rocks are recognized. The first is represented by the Toccone breccia (Late Cenomanian) belonging to the succession of the Balagne ophiolitic nappe, whereas the second, referred to as Volparone breccia (post-Jurassic), occurs as a slice at the base of the same nappe and displays no relationships with others lithologies. Both breccias can be regarded as deposits located close to their source area, as suggested by their sedimentological/stratigraphical features. Even if not coeval, these breccias were probably supplied by the same source area consisting of a metamorphic basement intruded by granitoids and covered by carbonate deposits. The recognized lithologies suggest that its source area was represented by a domain belonging to the Corsica continental margin. The Toccone and Volparone breccias are both characterized by mafic rocks, wich can be classified as transitional to tholeiitic within-plate basalts. It is noteworthy that clasts of ophiolites are completely lacking. The mafic rocks preserved as clasts in the Toccone and Volparone breccias are the remnants of a magmatism connected with the crustal thinning processes. This magmatism developed during the Late Permian-Jurassic before the opening of the Ligure-Piemontese oceanic basin. This magmatism probably developed in an area of the Hercynian basement that in the Late Jurassic became located in the oceanward side of the Corsica continental margin, i.e. the source area of the studied breccias.