The Longwoods Complex of Southland, New Zealand is part of an extensive terrane consisting of intrusives, volcanics, and sediments, which outcrops in the southern and north-western portions of the South Island. This terrane represents a volcanic arc which was active from Permian to Jurassic times (Grindley, 1958; Challis, 1968, 1969; Coombs et al., 1976). Between Pahia Point and Oraka Point on the southern coast of the South Island a section across the Longwoods Complex is well exposed and intrusives ranging in composition from ultrabasic cumulate rock, high-Al gabbro and gabbroic diorite to quartz diorite and granite outcrop. Two models have been considered for the origin of the rocks of the Pahia Point-Oraka Point section: (a) the rocks constitute one suite, the members of which are related by a crystal fractionation process; (b) the rocks constitute two suites which are not directly related.
The ultrabasic rocks, and quartz diorites are complementary and are derived from a high-Al gabbro parent by crystal fractionation involving pyroxene, olivine, plagioclase and hornblende, but considerations of viscosity and the geochemistry of the granite preclude derivation of the high-Si rocks by continuation of the crystal fractionation model. Furthermore, the quartz-diorites are of two types: xenolith bearing foliated quartz-diorites and xenolith deficient unfoliated types. The latter rock type appears to group with the gabbros on variation diagrams and partitioning of Ti between mica and amphibole supports the view that two distinct suites of rocks are involved: (a) a suite derived by fractional crystallization from a high-Al gabbro parent and consisting of cumulate ultramafic rocks, high-Al gabbro, gabbroic diorite and quartz-diorite; (b) a suite of foliated quartz diorites, formed by partial melting of lower crustal igneous rocks. The xenoliths in the foliated quartz-diorites represent modified residue left after partial melting. Melt and residue have unmixed to varying degrees during diapiric rise and a range of compositions has resulted.
The association of the two suites is tectonic. Gabbroic melts are generated in the lithosphere during plate subduction beneath a continental margin and rise of these melts into the lower continental crust results in partial melting and generation of quartz-diorite magmas.
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Price, R.C., Sinton, J.M. Geocheemical variations in a suite of granitoids and gabbros from Southland, New Zealand. Contr. Mineral. and Petrol. 67, 267–278 (1978). https://doi.org/10.1007/BF00381454
- Partial Melting
- Continental Crust