Petrology and Rb-Sr age of HP-LT Rocks from the Northern Carribean Plate Margin (Hispaniola) : implication for exhumation processes.

Philippe GONCALVES (1), Stéphane GUILLOT (2), Christian NICOLLET (1), Jean-Marc LARDEAUX (2) et Bernard MERCIER DE LEPINAY (3)

(1) Laboratoire de Géologie, CNRS, Université Blaise Pascal,Clermont-Ferrand, France
(2) Laboratoire de Dynamique de la Lithosphère, CNRS, UCB-ENS-Lyon, Villeurbanne, France
(3) CNRS, Géosciences Azur, Nice, France

Exhumation of high to ultra-high pressure metamorphic rocks was approached in most cases in orogenic belts (Caledonides, Alpes, Himalya...). In this collisional zone, early structures linked with subduction are partially obliterate. In contrast, outcrops of HP rocks in active subduction zone suggests a significant role of the subduction in the exhumation processes. The northen Carribean plate and especially the Samaná peninsula (Dominican Republic) is an example of transpressive area (southward dipping subduction of the north America plate with a left-lateral strike slip tectonics) where blueschists and eclogites are exhumed in subduction context

Paragenetic analysis and thermobarometric estimations confirm the presence of two different metamorphic units in the Samaná peninsula. Three complementary methods are used to evaluate P-T conditions : experimental stability fields of metamorphic assemblages, conventional thermobarometry and the Thermocalc computer program. A first low grade unit is characterised by the albite + lawsonite association and P-T conditions are about 7.5 ± 1 kbar and 320 ± 20 °C. The second unit overthrust the low grade unit to the north and blocks of eclogites which are commonly surrounded by blueschist rims are observed in the calcshists. The metamorphic evolution show an isothermal decompression at about 450 °C, from 14 kbar (LT eclogite facies) to 9 kbar into the epidote blueschist facies. The late evolution into the greenschist facies is contemporaneous with a syn-convergence extensive tectonic. It is indicated by conjugate normal faults perpendicular to the direction of convergence.

The Rb-Sr phengite - whole rock isochron from a high pressure unit eclogite give an age at 32 ± 2 My. This age is compatible with the current subduction of the noth american plate, active since about 85 to 90 My. Petrological analysis indicate that this age corresponds to the retrogression into the blueschist facies during the exhumation.

The Samaná metamorphic complex probably represents an accretionary wedge fragment carried on the active continental plate margin. This transpressive context, where minor collision occurs, show the importance of oblique subduction on exhumation processes. Moreover, the location of the eclogitic unit just north of the the left lateral strike-slip fault (Septentrional Fault Zone) suggest an important role of strike-slip fault during the exhumation.

Extension and strike-slip tectonics is a feature of the late orogenic stages (extensional collapse) and is also contemporaneous with the convergence in recent belts (Alpes, Himalaya), It appears they are present in the early orogenic stage, during subduction, before continental collision.