P-T-T CONSTRAINTS ON ACCRETION OF OPHIOLITES ALONG THE LLOYDS RIVER FAULT ZONE, SW NEWFOUNDLAND

The northeast-southwest trending Lloyds River Fault zone (LRFZ) separates the Annieopsquotch accretionary tract from the peri-Laurentian Dashwoods Block and its developing Notre Dame arc infrastructure to the northwest, and hence, forms a major structural boundary in the Newfoundland Appalachians. It comprises three different NW-dipping inter connected shear zones, which are dominated by variably amphibolitized gabbros with intrusive sheets of diorite and monzogabbro. The amphibolites are derived from the Annieopsquotch and Star Lake ophiolitic gabbros (~480 Ma), which formed in an infant arc/forearc setting outboard of the Dashwoods Block, and are exposed in the footwall of the fault zone. The monzogabbro and diorites are mantle-derived and have arc-signatures. The Lithoprobe seismic reflection profile indicates that the LRFZ extends to c. 10 km depth, where it appears to be cut off by the Red Indian line. Shear-sense indicators suggest the fault accommodated significant sinistral oblique underthrusting of the forearc crust beneath the Dashwoods Block. Time constraints on movement along the Lloyds River Fault zone are given by U/Pb ages of the diorite sheets, the monzogabbro, and stitching granites, as well as Ar/Ar ages of hornblendes from intrusive sheets and amphibolitized ophiolitic gabbros. The ages imply that the fault zone was active prior to 470 Ma, and continued to be active until at least 462 Ma. The southwestern part of the fault was reactivated during the Silurian in response to uplift of the Cormacks Lake complex. Hornblende-plagioclase thermometry and aluminum-in-hornblende barometry indicate that the footwall rocks experienced relatively low P- high T conditions during underthrusting, which is remarkable considering its setting. These conditions are best explained by intrusion of the various arc-like plutons along relatively shallow levels of the fault zone. The generation of these magmas during fore-arc subduction is enigmatic. One possible model involves stepping-back of the subduction zone to the east during and/or after the forearc subduction. Alternatively the arc-like magmatism is due to delamination of thickened arc lithosphere of the Notre Dame arc.