Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harsit Pluton, Eastern Turkey

We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (similar to 79 Ma) of the Harsit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO(2) = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO(2) = 54.95-56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb)(n) = 2.40-12.44] and display weak Eu anomalies (Eu/Eu* = 0.30-0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harsit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display I(Sr) = 0.70676-0.70708, epsilon(Nd)(79 Ma) = -4.4 to -3.3, with T(DM) = 1.09-1.36 Ga. The lead isotopic ratios are ((206)Pb/(204)pb) = 18.79-18.87, ((207)Pb/(204)Pb) = 15.59-15.61 and ((208)Pb/(204)Pb) = 38.71-38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir-Ankara-Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved similar to 65-75% of the lower crustal-derived melt and similar to 25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Harsit pluton formed in a subduction setting and that the back-arc extensional period started by least similar to 79 Ma in the Eastern Pontides.Geochemistry & GeophysicsMineralogySCI(E)33ARTICLE4467-48716

Metamorphism and deformation of mafic and felsic rocks in a magma transfer zone, Stewart Island, New Zealand

In the mingled mafic/felsic Halfmoon Pluton at The Neck, Stewart Island (part of the Median Batholith of New Zealand) some hornblende gabbros and diorites retain magmatic structures, whereas others show evidence of major changes in grain and inclusion shapes, and still others are amphibolite-facies granofelses with few or no igneous relicts. These mafic to intermediate magmas crystallized in felsic magma relatively quickly, with the result that most deformation occurred at subsolidus conditions. It is suggested that mafic-intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system. The metamorphism of the mafic rocks appears to be 'autometamorphic', in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling. Elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic-felsic magmas, into a dominantly transtensional, km-scale, outboard-migrating, magmatic shear zone that operated semi-continuously for between c. 140 and c. 130Ma. Complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard (southward) between c. 130 and c. 120Ma, associated with solid-state mylonitic deformation. Intermingled granitic rocks escaped metamorphism, because they remained magmatic to lower temperatures, and experienced shorter and lower-temperature subsolidus cooling intervals. However, the felsic rocks underwent relatively high-temperature solid-state deformation, as indicated by myrmekite replacing K-feldspar and chess-board subgrain patterns in quartz; locally they developed felsic mylonites. The felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals (quartz and biotite), whereas the mafic rocks mostly escaped subsolidus deformation, except in local high-strain zones of hornblende-plagioclase schist, because of their high proportion of relatively strong minerals (hornblende and plagioclase). We suggest that such contrasting microstructural features are diagnostic of long-lived syntectonic magma transfer zones, and contrast with the more typical complex, batholith-scale magma chambers of magmatic arcs

Interaction between felsic and mafic magmas in the Salmas intrusive complex, Northwestern Iran: constraints from petrography and geochemistry

The Salmas plutonic complex, in the northernmost part of Sanandaj-Sirjan Zone of Iran, provides evidence for magma interaction processes. The complex contains mafic-intermediate, hybrid and felsic rocks which intruded into the Paleozoic metamorphic complex. They show typical relationships described in many mafic–felsic mingling and mixing zones worldwide, such as mafic microgranular enclaves (in felsic and hybrid rocks), mafic sheets, and hybrid rocks. The mafic microgranular enclaves (MMEs) are characterized by fine-grained, equigranular and hypidiomorphic texture and some special types of microscopic textures, e.g., quartz xenocrysts, oscillatory-zoned plagioclase, small lath-shaped plagioclase in large plagioclase, spike zones in plagioclase and spongy-cellular plagioclase textures, rounded plagioclase megacrysts blade-shaped biotite, acicular apatite. The mafic sheets and MMEs in granites (MME-Gr), which indicated magma mingling structures, show ISr values and eNd(i) similar to diorites. The hybrid rocks and their mafic enclaves (MME-H) show isotope signatures similar to each other. Granites have isotope signatures [higher 87Sr/86Sr(i) (0.70788–0.71075) and lower eNd(i) ( 2.4 to 4.2)] distinct to those of the all rock types and MMEs. Major, trace and REE modeling show that hybrid rocks are generated via 40–60% mixing of mafic (dioritic) and felsic (granitic) end-members. All the geochemical data suggest that underplating of dioritic magma, which has been produced by fractional crystallization of gabbros, under the lower crust caused its melting to make felsic (granitic) magma. Injection of dioritic magma into the base of the felsic magma chamber and a limited mixing of two end-members, the lower crust-derived magma and mantle-derived melts, formed hybrid magma and their enclaves. Injections of new mafic magma pulses into hybrid magma generated mafic enclaves into them. The injections of denser dioritic magma pulses into a felsic magma chamber and spreading of it at the level of rheological contrast have been formed mafic sheets and MMEs in granites