Crystallisation of fine- and coarse-grained A-type granite sheets of the Southern Oklahoma Aulacogen, USA

A-type felsic magmatism associated with the Cambrian Southern Oklahoma Aula cogen began with eruption of voluminous rhyolite to form a thick volcanic c arapace on top of an eroded layered mafic complex. This angular unconformit y became a crustal magma trap and was the locus for emplacement of later su bvolcanic plutons. Rising felsic magma batches pending along this crustal m agma trap crystallised first as fine-grained granite sheets and then subseq uently as coarser-grained granite sheets. Aplite dykes, pegmatite dykes and porphyries are common within the younger coarser-grained granite sheets bu t rare to absent within the older fine-grained granite sheets. The older fi ne-grained granite sheets typically contain abundant granophyre. The differences between fine-grained and coarse-grained granite sheets can largely be attributed to a progressive increase in the depth of the crustal magma trap as the aulacogen evolved. At low pressures (< 200 MPa) a small increase in the depth of emplacement results in a dramatic increase in the solubility of H2O in felsic magmas. This is a direct consequence of the sha pe of the H2O-saturated granite solidus. The effect of this slight increase in total pressure on the crystallisation of felsic magmas is to delay vapo ur saturation, increase the H2O content of the residual melt fractions and further depress the solidus temperature. Higher melt H2O contents, and an e xtended temperature range over which crystallisation can proceed, both favo ur crystallisation of coarser-grained granites. In addition, the potential for the development of late, H2O-rich, melt fractions is significantly enha nced. Upon reaching vapour saturation, these late melt fractions are likely to form porphyries, aplite dykes and pegmatite dykes. For the Southern Oklahoma Aulacogen, the progressive increase in the depth of the crustal ma,sma trap at the base of the volcanic pile appears to refl ect thickening of the volcanic pile during rifting, but may also reflect em placement of earlier granite sheets. Thus, the change in textural character istics of granite sheets of the Wichita Granite Group may hold considerable promise as an avenue for further investigation in interpreting the history of this rifting event.