Granitoid pluton formation by spreading of continental crust: the Wiley Glacier complex, northwest Palmer Land, Antarctica.

The emplacement mechanism, geometry, and isotope geochemistry of plutons of the Wiley Glacier complex suggest that new continental crust grew by multiple injection of tonalitic dykes during dextral transtension in the Antarctic Peninsula magmatic are in Early Cretaceous times. The 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 Creswick Gap pluton and 140 +/- 5 Ma homblende gabbro. U-Pb dating of zircon and Ar-Ar dating of hornblende and biotite suggest that both granite s.l. plutons were emplaced between 145 and 140 Ma, but that extensional shearing was active from the time of emplacement until ca. lu Ma. The Bums Bluff pluton is chilled at its margin, and grades through mylonitised, porphyritic tonalite-granodiorite sheets and tonalite-granodiorite sheets with minor chilling, to a kilometre-scale body of coarse-grained, hypidiomorphic tonalite-granodiorite Co-magmatic microdiorite forms dykes and abundant synplutonic mafic enclaves. These dykes opened as echelon veins during episodic dextral shear and were deformed to trains of enclaves during continued normal-dextral shear. Pluton-marginal porphyritic and hypidiomorphic tonalite- granodiorite forms large, fault-hosted sheets emplaced progressively under extension with minor dextral shear. Kinematic indicators from pluton-marginal granite s.l. dykes suggest that early in pluton accretion, intrusive sheets cooled 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 slowly, emplacement switched from sheeting to in situ inflation with simple shear distributed across a broad zone prior to full crystallisation of magma. Cross-cutting relationships with the coeval, syn-extensional, Creswick Gap pluton suggest that the Bums Bluff pluton was emplaced in a steeper, second generation shear structure, like those in normal fault systems. This suggests that the Wiley Glacier complex was emplaced above the base of the brittle- ductile transition zone (15-18 km depth). 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 partial melts generated in the garnet-stability zone (40+ km depth). Addition of new crustal material by mafic underplating at the base of the crust and by redistribution of granitic s.l. and mafic, modified, underplated magma to mid- crustal levels along extensional shear zones as the are 'spread' were the primary mechanisms of crustal growth.

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