GRANITOID PLUTON FORMATION BY SPREADING OF CONTINENTAL-CRUST - THE WILEY GLACIER COMPLEX, NORTHWEST PALMER LAND, ANTARCTICA

The emplacement mechanism, geometry, and isotope geochemistry of pluto ns of the Wiley Glacier complex suggest that new continental crust gre w by multiple injection of tonalitic dykes during dextral transtension in the Antarctic Peninsula magmatic are in Early Cretaceous times. Th e suggested mechanism is analogous to basalt dyke injection during sea -floor spreading. During normal-dextral shear, the Bums Bluff pluton, a sheeted, moderately east-dipping, syn-magmatically sheared tonalite- granodiorite intruded syn-magmatically sheared quartz diorite of the C reswick Gap pluton and 140 +/- 5 Ma homblende gabbro. U-Pb dating of z ircon and Ar-Ar dating of hornblende and biotite suggest that both gra nite s.l. plutons were emplaced between 145 and 140 Ma, but that exten sional shearing was active from the time of emplacement until ca. lu M a. The Bums Bluff pluton is chilled at its margin, and grades through mylonitised, porphyritic tonalite-granodiorite sheets and tonalite-gra nodiorite sheets with minor chilling, to a kilometre-scale body of coa rse-grained, hypidiomorphic tonalite-granodiorite Co-magmatic microdio rite forms dykes and abundant synplutonic mafic enclaves. These dykes opened as echelon veins during episodic dextral shear and were deforme d to trains of enclaves during continued normal-dextral shear. Pluton- marginal porphyritic and hypidiomorphic tonalite-granodiorite forms la rge, fault-hosted sheets emplaced progressively under extension with m inor dextral shear. Kinematic indicators from pluton-marginal granite s.l. dykes suggest that early in pluton accretion, intrusive sheets co oled rapidly, with simple shear prior to full crystallisation changing to ductile simple shear during cooling. Kinematic indicators towards the pluton core suggest that as the pluton grew, and cooled more slowl y, emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation o f magma. Cross-cutting relationships with the coeval, syn-extensional, Creswick Gap pluton suggest that the Bums Bluff pluton was emplaced i n a steeper, second generation shear structure, like those in normal f ault systems. This suggests that the Wiley Glacier complex was emplace d above the base of the brittle-ductile transition zone (15-18 km dept h). The Bums Bluff pluton has Nd and Sr isotope values that range from mantle dominated (is an element of Nd-141 = +3.8, Sr-87/Sr-86(141) = 0.70468) to more crustally influenced (is an element of Nd-141 = -1.7, Sr-87/Sr-86(141) = 0.70652) This range probably represents different degrees of mixing between mantle-derived magma and lower crustal parti al melts generated in the garnet-stability zone (40+ km depth). Additi on of new crustal material by mafic underplating at the base of the cr ust and by redistribution of granitic s.l. and mafic, modified, underp lated magma to mid-crustal levels along extensional shear zones as the are 'spread' were the primary mechanisms of crustal growth.