CAUSES, MELT SOURCES AND PETROLOGICAL EVOLUTION OF ADAKITIC MAGMATISM IN NW TURKEY

References: 12th International Multidisciplinary Scientific GeoConference, www.sgem.org, SGEM2012 Conference Proceedings/ ISSN 1314-2704, June 17-23, 2012, Vol. 1, 17 - 24 pp

ABSTRACT

We document the occurrence of adakite like porphyries in NW Anatolia and report a
comprehensive data set (petrography, major-trace element geochemistry, Sr-Nd isotope
compositions and 40Ar/39Ar radiometric ages) from them in order to evaluate the
causes, melt sources and petrological evolution of Adakitic magmatism in NW Turkey.
A number of Eocene plutons and porphyries emplaced into the Izmir-Ankara suture
zone (IASZ), which represents the collision zone between Anatolide-Tauride platform
and Sakarya continent.
Adakite-like porphyries form stocks, sills and dikes that are spatially and temporally
associated with Eocene plutons emplaced into ophiolitic and blueschist rocks of
Tavşanlı zone. Petrographically, they can be classified as porphyritic microgranite, and
microgranodiorite. They display holocrystalline, microgranular porphyritic textures, and
composed mainly of plagioclase, K-Feldspar, quartz, hornblend and biotite.
Geochemically, adakitic rocks have high SiO2 (69.43-63.80), Al2O3 (15.44-16.64)
contents, high Sr/Y (113.39-214.92) and La/Yb(n) (35.33-80.18) ratios and low Y(6.2-
11.5), Yb (1.07-0.54) contents, and display enrichments of LILE and LREE , depletion
of HFSE and lack of Eu anomaly. Our 40Ar/39Ar ages obtained from adakite-like
porphyries (53.7 to 54.0 Ma ) indicate that adakitic magmatism occured during the
Early Eocene in NW Anatolia, as documented in Eastern Pontides (NE Turkey).
They have moderately enriched 87Sr/86Sr (54) (0.70620–0.70660) and low εNd (54)
(−2.73 to 0.59). Geochemistry and isotope compositions of adakitic porphyries indicate
that they most likely to be produced by partial melting of garnet-bearing lower crustal
lithologies and its interaction with mantle melt. Assimilation combined with fractional
crystallization (AFC) was played an important role during the evolution of these melts
at shallower crustal levels.
Geochronology, geochemistry and geology of Adakitic porphyries and geology of the
NW Anatolia indicate that adakitic magmatism was not formed above an actively
dehydrating subducted slab. Instead, our observations are consistent with a magmatism
that is more typical of syn-convergent extensional tectonic settings. Formation of
adakitic melts require increased heat flow at about 54 Ma that might have occurred by
slab break-off.

Keywords: Petrology, Adakite-like porphyries, NW Turkey, Eocene, Slab break-off