EPISODIC RAPAKIVI MAGMATISM DUE TO DISTAL OROGENESIS?: CORRELATION OF 1.69 - 1.50 GA OROGENIC AND INBOARD, "ANOROGENIC" EVENTS IN THE BALTIC SHIELD

Abstract

The temporal overlap of 1.69 1.55 Ga westward growth of the Baltic Shield and voluminous, episodic, 1.65 1.50 Ga rapakivi magmatism in the Svecofennian domain to the east has long been recognized. New U-Pb data from southwest Sweden indicate that this westward growth occurred in at least three distinct stages involving convergent-margin magmatism (Gothian orogenesis) at 1.69 1.65, 1.62 1.58, and 1.56 1.55 Ga. This improved resolution along the margin reveals an interesting temporal correlation between these events and the intracratonic rapakivi magmatism. Each stage of convergent-margin magmatism was echoed by a phase of bimodal rapakivi magmatism at 1.65 1.62, 1.58 1.56, and 1.55 1.50 Ga. In addition to this temporal correlation, rapakivi suites form distinct north-trending arrays that are subparallel to, but 500 1500 km east of, the active margin. These temporal and spatial links suggest that recurring subduction along oceanward-stepping zones provided first-order control(s) on episodic mantle melting and consequent bimodal rapakivi magmatism in distal, inboard settings. At the very least, emerging constraints from Baltica do not support previous, purely anorogenic models for the 1.65 1.50 Ga rapakivi magmatism or models that singularly implicate lingering effects from the preceding Svecofennian orogeny.