Petrology,geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence,eastern Rhodope,Bulgaria

摘要

In the eastern Bulgarian Rhodope,mafic extrusive rocks and underlying greenschists are found in the Mesozoic low-grade unit,which represents the northern extension of similar sequences including the Evros ophiolites in Thrace(Greece).Both rock types define a suite of low-Ti tholeiitic basalts to transitional boninitic basaltic andesites and andesites and associated metapyroclastites(greenschists),intruded at its base by diorite dikes of a boninitic affinity.Mafic lavas and greenschists display large ion lithophile element(LILE)enrichment relative to high-field strength elements(HFSE),flat REE patterns of a slight light REE depletion,a strong island arc tholeiite(IAT)and weak MORB-like signature.All these rocks are characterized by negative Nb anomalies ascribed to arc lavas.They have positive epsilon Nd_i values in the range of +4.87 to +6.09,approaching the lower limit of MORB-like source,and relatively high(~(207)Pb/~(204)Pb)_i(15.57-15.663)at low(~(206)Pb/~(204)Pb)_i(18.13-18.54)ratios.The Nd isotopic compositions coupled with trace element data imply a dominantly depleted MORB-like mantle source and a contribution of subduction modified LILE-enriched component derived from the mantle wedge.The diorite dike has a low epsilon Nd_i value of-2.61 and is slightly more Pb radiogenic(~(207)Pb/~(204)Pb)_i(15.64)and(~(206)Pb/~(204)Pb)_i(18.56),respectively,reflecting crustal contamination.Petrologic and geochemical data indicate that the greenschists and mafic extrusive rocks represent a magmatic assemblage formed in an island arc setting.The magmatic suite is interpreted as representing an island arc-accretionary complex related to the southward subduction of the Meliata-Maliac ocean under the supra-subduction back-arc Vardar ocean/island arc system.Magmatic activity appears to have initiated in the north during the inception of the island arc system by the Early-Middle Jurassic time in the eastern Rhodope that most likely graded to back-arc spreading southwards as represented by the Late Jurassic MORB-type Samothraki Island ophiolites.This tectonic scenario is further constrained by paleotectonic reconstructions.The arc-trench system collided with the Rhodope in the Late Jurassic times.