Geology, geochemistry and geochronology of the Cretaceous Lascano East intrusive complex and magmatic evolution of the Laguna Merin basin, Uruguay

摘要

The Cretaceous Laguna Merin rift basin in southeastern Uruguay formed during the opening of the southern Atlantic Ocean, and is filled with igneous rocks that we characterize here by field, age, geochemical, and geophysical studies. Initial bimodal magmatic suites (similar to 134 to 128 Ma) include volcanic rocks and intrusions of the sub-alkaline Parana-Etendeka large igneous province as well as mildly alkaline (transitional) suites. This was followed by younger (similar to 128 to <127 Ma) alkaline gabbro and trachyte intrusions. Ten units of sub-alkaline, transitional, and alkaline rocks can be distinguished by whole rock composition and Nb/Zr ratios, which increase from 0.05 to 0.37 as alkalinity increases. The early sub-alkaline basalts are widespread throughout the Laguna Merin basin, however, later sub-alkaline rhyolites and more alkaline rocks are localized near four similar to 20 to 30 km diameter ring complexes named Valle Chico, Lascano West, Lascano East and San Luis. Each of the four complexes has a positive gravity anomaly (similar to 80 mGals) and cospatial aeromagnetic anomaly (similar to 1200 nT) with an abrupt margin interpreted as a ring fault. Modeling suggests that the gravity anomalies result from shallowly emplaced and dense gabbro dikes and sills, and additional underlying and more voluminous intrusions extending to more than 5 km depth. The magnetic anomalies correspond to syenites exposed at Valle Chico and sub-alkaline basalts and gabbro dikes at Lascano East. Trace element modeling indicates that the sub-alkaline magmas formed via large degrees of partial melting of shallow mantle sources, whereas the alkaline magmas formed via lower degrees of partial melting of deeper asthenospheric mantle, with progressive deepening of the mantle sources through time. Sr and Nd isotopic compositions of Nd-144/Nd-143 < 0.5126 and Sr-87/Sr-86 > 0.705 evidence crustal contamination even for the most primitive magmas. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.