Subduction initiation for the formation of high-Cr chromitites in the Kop ophiolite, NE Turkey

Abstract

The Kop ophiolite in NE Turkey is a forearc fragment of Neo-Tethys ocean, consisting mainly of a paleo-Moho transition zone (MTZ) and a harzburgitic upper mantle unit. Locally, the Kop MTZ contains cumulate dunites and high-Cr chromitites (Cr# up to ca. 79), which are cut by pyroxenites. Dunites and chromitites in the MTZ have REE concentrations that are 1-2 orders of magnitude lower than those of chondrite; they are either depleted in LREE or have concave REE shapes. The LREE depleted patterns are interpreted to reflect production of cumulate rocks by magmas derived from a depleted mantle, the concave patterns the modification of these rocks by LREEenriched fluids. Clinopyroxenes from pyroxenites are diopsidic and characterized by high Mg#s (ca. 92-96) and high CaO contents (ca. 24-25 wt.%); their Al 2 O 3 contents (1.0-3.0 wt.%) fall between those of clinopyroxenes in N-MORB and komatiite/boninite, suggesting that the parental melts originated from more refractory mantle than abyssal lherzolites. However, these clinopyroxenes display LREE depleted patterns consistent with those of clinopyroxenes in abyssal lherzolites, indicating their genetic connection with decompression melting of asthenosphere. The cross-cutting relationship between pyroxenite veins and chromitiferous rocks suggests that depleted mantle remained beneath the proto-forearc after chromitite formation; it had not been significantly modified by slab-derived components and continued interacting with the upwelling asthenosphere until pyroxenite crystallization. This study provides a temporal constraint on the formation of high-Cr chromitites; they possibly began to be produced during the transition between early and late proto-forearc spreading, during which subduction dehydration had not well developed.

Mineralogical Evidence for Partial Melting and Melt-Rock Interaction Processes in the Mantle Peridotites of Edessa Ophiolite (North Greece)

“…It is argued above that the Al-spinels from the lherzolite are residues after low to moderate degrees of partial melting in a spreading environment. The origin of the residual Cr-spinels of the harzburgites (including the cpx-bearing ones) with Cr# clustering between 30.9 and 58.5 ( Figure 5) require higher degrees of melting (up to 19%), which are consistent with an arc setting and hydrous conditions [82]. The existence of the subhedral to anhedral chromites (Cr#-poorer-Mg#-richer in Figure 5) indicate a chemical modification of Cr-spinels after interaction with a Cr-rich melt and is in line with this interpretation.…”

Post-magmatic processes recorded in bimodal chromitites of the East Chalkidiki meta-ultramafic bodies, Gomati and Nea Roda, Northern Greece

“…However, Rui et al (2022) argue that these diopsides are not syngenetic with the chromitites. Moreover, Zhang et al (2016) attribute LREE enrichments in clinopyroxene hosted in subduction-initiation related dunite and Crchromitites (demonstrating concave spoon-shaped patterns) to their post-magmatic equilibration with metasomatic fluids derive from the subducted slab. Therefore, the diopside hosted in G1 chromitites is also a metasomatic affected phase.…”

Making Cratonic Lithospheric Mantle

“…The Rudnick et al, 1993;Simon et al, 2003;Wittig et al, 2008;Lee et al, 2011, and reference therein) and forearc peridotites (Bénard et al, 2017;Ohara & Ishii, 1998;Parkinson & Pearce, 1998;Pearce et al, 2000;Savov et al, 2005;Savov et al, 2007). SSZ ophiolitic peridotites (Batanova et al, 2011;Büchl et al, 2002;Habtoor et al, 2017;Khalil & Azer, 2007;Khedr & Arai, 2017;Liu et al, 2016;Mahéo et al, 2004;Marchesi et al, 2009;Moghadam et al, 2015;O'Driscoll et al, 2012;Pagé et al, 2009;Rajabzadeh & Dehkordi, 2013;Saka et al, 2014;Uysal et al, 2012;Uysal et al, 2015;Uysal et al, 2016;Zhang et al, 2016;Zhou et al, 1996), abyssal peridotites (Aumento & Loubat, 1971;Brandon et al, 2000;Cannat et al, 1995;Casey, 1997;Coogan et al, 2004;Godard et al, 2008;Harvey et al, 2006;Miyashiro et al, 1969;Niu, 2004;Niu & Hékinian, 1997;Paulick et al, 2006;Snow & Dick, 1995;…”

“…Data sources: Cratonic peridotite xenoliths (Carlson et al, ; Rudnick et al, ; Simon et al, ; Wittig et al, ; Lee et al, , and reference therein) and forearc peridotites (Bénard et al, ; Ohara & Ishii, ; Parkinson & Pearce, ; Pearce et al, ; Savov et al, ; Savov et al, ). SSZ ophiolitic peridotites (Batanova et al, ; Büchl et al, ; Habtoor et al, ; Khalil & Azer, ; Khedr & Arai, ; Liu et al, ; Mahéo et al, ; Marchesi et al, ; Moghadam et al, ; O'Driscoll et al, ; Pagé et al, ; Rajabzadeh & Dehkordi, ; Saka et al, ; Uysal et al, ; Uysal et al, ; Uysal et al, ; Zhang et al, ; Zhou et al, ), abyssal peridotites (Aumento & Loubat, ; Brandon et al, ; Cannat et al, ; Casey, ; Coogan et al, ; Godard et al, ; Harvey et al, ; Miyashiro et al, ; Niu, ; Niu & Hékinian, ; Paulick et al, ; Snow & Dick, ; Stephens, ), and MOR ophiolitic peridotites (Austrheim & Prestvik, ; Beccaluva et al, ; Godard et al, ; Hanghøj et al, ; Iyer et al, ; li & lee, ; Liu et al, ; Maaløe, ; Marchesi et al, ; Ottonello et al, ; Uysal et al, ; Yumul Jr et al, ). SSZ = suprasubduction zone; MOR = mid‐ocean ridge.…”