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

Abstract

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 arc in Early Cretaceous times. The suggested mechanism is analogous to basalt dyke injection during sea-floor spreading. During normal-dextral shear, the Burns 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 hornblende 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. 127 Ma. The Burns 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 Burns 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 Burns Bluff pluton has Nd and Sr isotope values that range from mantle dominated ( ϵNd 141 = +3.8, 87Sr 86Sr 141 = 0.70468 ) to more crustally influenced ( ϵNd 141 = −1.7, 87Sr 86Sr 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 arc ‘spread’ were the primary mechanisms of crustal growth.