Precambrian Mafic Magmatism in the Himalayan Mountain Range

Precambrian mafic magmatic rocks are restricted to the western Himalayas from Himachal in the west through Garhwal to Kumaun in the east. They occur as low-grade volcano-sedimentary sequences of the Lesser Himalaya in the form of volcanic flows intercalated with continentally derived sediments and also as dykes occurring as feeders and some traversing through these sequences. Thus, all the Precambrian mafic magmatic rocks are not contemporary but may represent more than one magmatic episode. However, it is not very clear how much is the gap between the individual magmatic phases. In addition to the low-grade sequences, mafic magmatic rocks are also recorded from the Chail, Jutogh and Vaikrita Groups and their equivalents in Garhwal and Kumaun regions. In these three sequences the mafic magmatic rocks are represented by amphibolites which occur as shapeless bodies, boudins, disrupted dykes/sills varying in size from 1-2 m in thickness and 3-5 m in width. The basic rocks of all the units consist of clinopyroxene (augite and ferro-augite) and plagioclases as the dominant phases with minor olivine (now serpentine). Commonly observed secondary phases include amphiboles, chlorite, opaques, epidote, quarts, biotite and titanites.

Geochemical data indicates that these rocks are compositionally basalt and basaltic - andesites. They have typical tholeiitic lineage with Fe-enrichment and are classified as low-Ti tholeiites. Rare earth elements (REE) and incompatible trace elements data indicate enriched light rare earth element (LREE) - large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE) characteristics for the mafic rocks of the lower grade Lesser Himalayan sequences, Chail and Jutogh Groups. The mafic rocks of Vaikrita Group, on the other hand show nearly flat REE and incompatible trace elements patterns with distinct positive Sr-anomaly, which is very different from the other groups. These characteristics indicate derivation of Vaikrita mafic rocks from different sources compared to the other samples. Lithological association and geochemical characteristics indicate derivation of the mafic magmatic rocks from enriched lithospheric mantle sources by varying degrees of partial melting followed by clinopyroxene ± minor olivine ± plagioclases ± opaques in a rift tectonic environment.