Petrology And Tectonic Evolution Of The RasKoh Ophiolite In Western Pakistan

The RasKoh ophiolite,located in the RasKoh Range of western Pakistan,comprises the two ophiolitic massifs,Sorap Massif(western)and Bunap Massif(eastern)of the RasKoh range.It forms one important segment of the vast Alpine-Himalayan orogenic belt(AHOB),which is a key to tracking the tectonic origin of the Neo-Tethys.In contrast to their equivalents in Yarlung–Zangbo surure zone of southern Tibet,Iran,Oman,and adjacent Mediterranean regions,very few data are available for the Pakistan ophiolites.Our recently completed study of the RasKoh ophiolite in western Pakistan reveals that these ophiolites are represented by well-developed metamorphic sole rocks,serpentinized peridotites,ultramafic rocks,subordinate gabbros,minor plagiogranite,conglomerates-bearing radiolarian chert,and pillow lavas,which shows certain tectonic mèlange in nature.This thesis study serves as the first record on geological field observations,geochemical descriptions,and geochronology of peridotites,ultramafic and mafic-silicic rocks in the RasKoh ophiolitic belt.This study describes their geological,mineralogical,geochemical,and geochronological features and reveals their petrogenesis and geodynamic evolution processes.Field observations,confirmed by thin-section study,identified three kinds of peridotite;lherzolite,harzburgite,and dunite.The mineral compositions of the peridotites(olivine,spinel,and pyroxene)suggest that these ultramafic rocks represent the refractory residues of mantle peridotites after?12 to 25%partial melting.This interpretation is supported by variable contents of major elements and middle(MREE)and heavy(HREE)rare earth elements.However,the contents of water-soluble elements such as Rb,Ba,Sr,and light rare earth elements(LREE)indicate infiltration of fluid into these depleted peridotites.Synthesizing available geological information with our new geochemical data,we suggest that the RasKoh ophiolite is a fragment of a former Triassic to Jurassic oceanic plate that became a mantle wedge above an intra-ocean subduction zone during the Cretaceous.Fluid-mobile material from the subducted plate re-enriched the depleted peridotites of the RasKoh island arc.The ophiolite was emplaced during the collision between the RasKoh island arc to the south and the Chagai continental arc to the north leading to exhumation of the re-enriched peridotites.The geochemistry of mafic and silicic rocks including gabbroic rocks and plagiogranites is similar to the typical island arc tholeiites(IAT).All samples display supra-subduction zone(SSZ)geochemical affinities such as enrichment in the large ion lithophile elements(LILEs)and depletion in the high field strength elements(HFSEs,coupled with high and positive whole-rock?Nd(t)values(+6.4–+7.6).These geochemical features suggest a N-MORB source and progressively increased subduction inputs to the mantle source through time in a back-arc setting.The zircons from two anorthositic-gabbro samples exhibiting high Th/U ratios and magmatic textures,yield concordant mean U-Pb ages of 63.6±1.1 Ma(n=17,MSWD=1.7),and 65.5±1.7 Ma(n=15,MSWD=0.61),respectively.These relatively younger zircon ages are interpreted as the RasKoh ophiolite was formed in the Late Cretaceous to Early Paleocene.Our new data provide significant constraints about the Neotethyan convergence between Eurasian(Afghan Block)and Indian Plates and the presence of backarc oceanic-crust in RasKoh arc at the southern margin of Afghan Block in Late Cretaceous to Early Paleocene.Small,isolated outcrops of metamorphic sole rocks of the RasKoh ophiolite are exposed southeast of the RasKoh Arc range.These crop out as thin slices beneath the sheared serpentinites,and consist of amphibolites,epidote-bearing amphibolites,and muscovite-rich amphibolites with granoblastic,grano-nematoblastic,and porphyroblastic textures.Geochemically,amphibolites present two compositional groups.Group-I amphibolites are alkaline and enriched in light rare earth elements(LREE;La _N/Yb _N=11.47–11.60)with Sr-Nd isotopic compositions of(⁸⁷Sr/⁸⁶Sr)_i=0.705481–0.706128,(¹⁴³Nd/¹⁴⁴Nd)_i=0.512668–0.512672,and lower values of?Nd_(t)(+2.24–+2.32).Whilst,Group-II amphibolites are tholeiitic with LREE-depleted signatures(La _N/Yb _N=0.88–1.31),(⁸⁷Sr/⁸⁶Sr)_i=0.706103–0.706303,(¹⁴³Nd/¹⁴⁴Nd)_i=0.512851–0.512936 and higher values of?Nd_(t)(+5.81–+7.47).Primitive mantle-normalized and other geochemical diagrams indicate that the protoliths for Group-I amphibolites are oceanic island basalts(OIB).Conversely,Group-II amphibolites are more akin to subduction-related arc tholeiites(IAT)with geochemical signatures,slightly enriched in large ion lithophile elements(LILE)and with negative Nb-Ta anomalies.The geothermobarometric results estimate that these rocks were metamorphosed at amphibolite facies at 690?and 6.5kbar.Alkaline volcanism is well documented from the other Mesozoic ophiolites of Neo-Tethyan suture zones.We suggest that the oceanic island basalts accreted to the base of the obducted plate at an early stage,whereas the IAT basalts metamorphosed and accreted to the base of the RasKoh ophiolite at a later stage.The OIB-and IAT-type magmas as protolith of amphibolites suggest that the RasKoh ophiolite emplaced in a supra-subduction zone(SSZ)-type forearc in Late Cretaceous(ca.68–64 Ma).Synthesizing comprehensively the new and previously available geological,petrographic,mineralogical,geochemical,and geochronological data,the regional tectonic evolution of the RasKoh ophiolite belt in western Pakistan can be summarized into a three-stage genetic model.(a)Formation of the Neo-Tethyan Ocean(Triassic to Jurassic):No radioactive isotropic data is available to constrain the precise timings of these peridotites.However,the earliest arc-like volcanics along the southern margin of Eurasia are Late Triassic to the earliest Jurassic in age.This indicates the Neo-Tethys formed before the Late Triassic:the formation of ophiolite rock assemblages(peridotites).Some investigators have suggested that oceanic floor rifting in Neo-Tethys began as early as the Permian.(b)Double-subduction and accretion system of the Neo-Tethyan lithosphere(Late Jurassic to Cretaceous):The subduction of Neo-Tethyan lithosphere beneath the southern margin of the Afghan Block gave rise to the Jurassic to Cretaceous Chagai continental margin arc,and this continental arc is not limited to the Chagai region but is widespread along the entire southern margin of Eurasia.Meanwhile,an intra-ocean subduction zone developed within the Neo-Tethyan Ocean,south of the ocean-continent subduction zone,forming a double-subduction system,and the intra-ocean subduction led to formation of the Cretaceous RasKoh island arc.The deep subduction of the early ocean ridge ophiolite formation occurred exhumation and reattachment and developed at the same time that the intra-oceanic arc magmatism and the substantial transformation of the early oceanic crust,the partial melting of rocks and serpentinization of peridotite,the formation of metamorphic sole rocks and plagiogranite,gabbro dikes.This is also a new mechanism of the ophiolite genesis proposed by us during this study.(c)Arc-arc collision of the Chagai and the RasKoh arcs(Late Cretaceous to Early Eocene,?68?46 Ma):The oceanic basin between the Chagai and the RasKoh arcs disappeared in the late Late Cretaceous due to continued subduction,resulting in the arc–arc collision between the Chagai continental arc and the RasKoh island arc.Continued subduction of the oceanic plate,south of the RasKoh Arc,gave rise to the Early Eocene Chagai-Raskoh Arc.Meanwhile,the oceanic plate south of the RasKoh Arc continued to subduct beneath the arc–arc collisional belt,giving rise to the Eocene Chagai–RasKoh Arc.It is reasonable to suggest that the arc–arc collision and related deformation caused some parts of the mantle wedge to be thrust onto over the surficial arc sequence where they mingled with the arc-volcanic rocks.Some depleted peridotites of the mantle wedge were re-enriched during subduction prior to the arc-arc collision.The depletion and re-enrichment processes are well recorded in the RasKoh peridotites.