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

• Four 20 km diameter intrusive complexes identified in the Merín basin (134–127 Ma)
• The complexes are defined by positive gravity and magnetic anomalies.
• Early large degrees of partial melt evolved to later low degree of partial melt.

The Cretaceous Laguna Merín 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 (~ 134 to 128 Ma) include volcanic rocks and intrusions of the sub-alkaline Paraná–Etendeka large igneous province as well as mildly alkaline (transitional) suites. This was followed by younger (~ 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 Merín basin, however, later sub-alkaline rhyolites and more alkaline rocks are localized near four ~ 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 (~ 80 mGals) and cospatial aeromagnetic anomaly (~ 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 144Nd/143Nd < 0.5126 and 87Sr/86Sr > 0.705 evidence crustal contamination even for the most primitive magmas.

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