Petrogenesis of the Murrumbidgee Batholith, A.C.T.

Abstract

The Murrumbidgee Batholith, A.C.T., is an example of a batholith containing abundant metasedimentary xenoliths. It is a composite, intrusive batholith with an outcrop area of 1400 km² and a range in composition from tonalite to leucogranite. Its intrusions can be classified into three natural groups on the basis of textural, modal, and chemical criteria. Detailed chemical studies of variation within the batholith, supported by field and petrographic observations, indicate that all three granite groups evolved from parental magma of adamellite composition, generated by partial melting of psammopelitic rocks within the Tasman Geosyncline. Fractional crystallization of this parental magma, after emplacement, gave rise to complementary adamellites (‘uncontaminated granite group’) and leucogranites (‘leucogranite group’). Subsequent emplacement of parental magma which had reacted extensively with relict solid material (‘xenoliths’) at, or near, its source gave rise to granodiorites and tonalites (‘contaminated granite group’). The Murrumbidgee Batholith belongs to one of three types (‘foliated type’) of batholiths long recognized in the Tasman Geosyncline. Its origin is closely related to one other type (‘gneissic type’) but quite different to that of the third type (‘massive type’), recently interpreted as forming by partial melting of basic rocks. It is thought that high water contents and endothermic assimilative reactions would characterize acid magmas generated by partial melting of sedimentary rocks, whereas low water contents and exothermic assimilative reactions would characterize acid magmas formed by partial melting of basic rocks. These points are invoked to explain the mesozonal and catazonal emplacement of the ‘gneissic’ and ‘foliated’ batholiths in contrast to the epizonal emplacement of the ‘massive’ type.