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 eNdi
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.
(c) 2007 Elsevier B.V. All rights reserved.