MAGMATISM DURING ARC-CONTINENT COLLISION IN TAIWAN AND SOUTH MAYO: IMPLICATIONS FOR THE ORIGIN OF THE CONTINENTAL CRUST

The collision of intra-oceanic island arcs with passive continental margins is likely the principle mechanism by which the continental crust has grown, at least during later Precambrian and Phanerozoic times. It may also be the only mechanism for turning a passive into an active continental margin. We have been exploring the tectonic and magmatic evolution of arcs during arc-continent collision in Taiwan and in the Caledonides of Ireland in order to understand how this process transforms arc crust into material close in composition to the continental crust. The Irish arc system shows a mafic intra-oceanic subduction system with positive e_Nd and depleted light rare earth element (LREE) patterns evolving to more negative e_Nd and higher LREE-enrichment during the initial subduction of a distal passive margin. During the “hard” collision of arc and margin a 10 m.y. phase of volcanism involves major recycling of the crust. However, because LREE enrichment of both volcanics and mid crustal gabbros during this phase exceeds the average continental crust, the LREE enrichment cannot simply reflect crustal recycling. Addition of such material to the depleted arc crust opens the possibility that a mixture of the two could yield a net product close to the continental average. The high LREE enrichment of the syn-collisional magmas may partially reflect deep melting in the presence of garnet, but is mostly the product of fractional crystallization in the lower crust. Extensional orogenic collapse, resulting in rapid exhumation, marks the end of the collisional phase and a switch to volcanism with high but variable high field strength element and LREE enrichment. These magmas are often also alkalic. In Taiwan extension culminates in the formation of the Okinawa Trough (analogous to the South Mayo Trough), whose propagation approximates the rate of arc migration along the margin. We consider this basin not as a classic backarc basin behind a steady-state Ryuku Arc, but instead as representing orogenic collapse at the point of subduction polarity reversal, when the Ryuku Arc is first formed. Orogenic collapse may be triggered in part by the gravity driven loss of the dense arc lower crust. Seismic surveys do not show high velocity, reflective ultra-mafic arc lower crust, such as known from the active Aleutian and Marianas Arc under the Iapetus suture zone, consistent with its loss during collision in Ireland. Loss of the lower crust also helps push the average accreted crust to more silicic values. Subduction polarity reversal does not require a break of the original (South China Sea) subducting oceanic slab in order to accommodate the introduction of a new (Philippine Sea) oceanic slab under the reversed polarity margin. Instead the progress can be achieved by a progressive tearing and retreat of the South China margin lithosphere, creating a lithospheric gap into which the oceanic Philippine Sea Plate is inserted as Luzon continues its progressive march westward along the passive margin. There is no need to snap the original subducting slab in order to permit the reverse polarity subduction to start. Arc-continental collision can be considered as a continuous, steady state process. The entire process from initial margin subduction to the end of collapse and extension does not exceed ~15 m.y. in South Mayo, probably less in Taiwan.