Trace element geochemistry and genesis of Precambrian sub-alkaline mafic dikes from the central Indian craton: evidence for mantle metasomatism

摘要

Precambrian mafic dikes transect the Archaean granite gneisses of the southern Bastar craton, Central India. On the basis of high-field strength elements, the Bastar mafic dike swarms were sub-divided into an earlier BD1 and a later BD2 system. The BD1 set comprises amphibolites, whereas the BD2 set consists of dolerites/metadolerites. On the basis of field relationships arid available age data for enveloping granite rocks, the BD1 dikes are regarded as middle Archaean in age and BD2 dikes are Paleoproterozoic (～ 1.8 Ga). The silica and total-alkalis contents of dikes of both swarms classified them mostly as sub-alkaline basalts. The BD1 dikes are low Ti + Fe and high-Mg olivine to quartz normative rocks, whereas the BD2 dikes are predominantly quartz normative with relatively high Ti + Fe and low-Mg contents. The high-field strength element concentrations are relatively higher in the BD2 dikes than in the BD1 dikes for comparable values of MgO and/or Cr. Judging from the geochemical data, these dikes probably were not contaminated by crustal components. The geochemistry and field observations clearly suggest a continental rift environment for the emplacement of both BD1 and BD2 dikes. Rare-earth element patterns and compatible-incompatible trace element modelling suggest that the two swarms were derived from distinctly different sub-alkaline basaltic magmas. The middle Archaean BD1 dikes were derived from ～ 15-20% batch melting of a depleted Iherzolite mantle source; in contrast, the Paleoproterozoic BD2 dikes were derived from ～ 7-10% batch melting of a relatively enriched mantle source. A post Archaean increase in the thickness of metasomatized mantle lithosphere is the probable cause of mantle enrichment.