U-Pb geochronology and isotopic geochemistry of adakites and related magmas in the Ediacaran arc section of the SW Iberian Massif: The role of subduction erosion cycles in peri-Gondwanan arcs

Abstract

In the peri-Gondwanan Ediacaran arc section outcropping in the SW of the Iberian Massif (Mérida region), changes in the subduction angle and subduction rate, variable participation of highly modified mantle sources together with the role of old crustal material and juvenile mafic rocks driven by the incoming slab, occur with a recognizable cyclicality and can be traced according to the geochronology, geochemistry and isotopic sources of the magmatic bodies. Our results support the initial existence of a mantle wedge highly modified due to percolation of significant volumes of variably old crustal materials, whose isotopic sources are almost identical to those found in the oldest known metasedimentary sequence described in SW Iberian Massif, the Serie Negra Group. Old crustal sources, analogous to those of this series were introduced to high depths by common subduction an also probably by significant subduction erosion. During the oldest period recorded in the study arc section, between c. 602 and c. 550 Ma, subduction episodes that involve high rates of crustal material along with an eventual incorporation of mafic and ultramafic materials from the incoming oceanic plate, favoured by low subduction angles, lead to the generation of more silicic magmas with adakitic geochemical affinity and isotopic signature (Nd-Sr) with crustal tendency. However, an increase in the subduction angle and change to a roll-back stage dated at c. 540 Ma, caused significant decrease in the access of cortical material to the subduction channel, favouring generation of typical calc-alkaline magmas derived from the modified mantle wedge, which is also consistent with more juvenile Nd-Sr ratios. The correlation found between the magmatic events and the involved tectonic processes suggests that subduction erosion mechanisms have been very likely underestimated as active dynamic processes along the peri-Gondwana margin during at least Neoproterozoic to Early Cambrian times.