Construction of the granitoid crust of an island arc part I: geochronological and geochemical constraints from the plutonic Kohistan (NW Pakistan)

We present major and trace element analyses and U-Pb zircon intrusion ages from I-type granitoids sampled along a crustal transect in the vicinity of the Chilas gabbronorite of the Kohistan paleo-arc. The aim is to investigate the roles of fractional crystallization of mantle-derived melts and partial melting of lower crustal amphibolites to produce the magmatic upper crust of an island arc. The analyzed samples span a wide calc-alkaline compositional range (diorite-tonalite-granodiorite-granite) and have typical subduction-related trace element signatures. Their intrusion ages (75.1±4.5-42.1±4.4Ma) are younger than the Chilas Complex (~85Ma). The new results indicate, in conjunction with literature data, that granitoid formation in the Kohistan arc was a continuous rather than punctuated process. Field observations and the presence of inherited zircons indicate the importance of assimilation processes. Field relations, petrographic observations and major and trace element compositions of the granitoid indicate the importance of amphibole fractionation for their origin. It is concluded that granitoids in the Kohistan arc are derivative products of mantle derived melts that evolved through amphibole-dominated fractionation and intra crustal assimilatio

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.National Science Foundation (U.S.) (Grant EAR 6920005

Changing Sources of Magma Generation Beneath Intra-Oceanic Islands Arcs: An Insight From the Juvenile Kohistan Island Arc, Pakistan Himalaya

The Kohistan arc, situated in the Pakistan Himalaya, is a Cretaceous intraoceanic island arc which was initiated during the northward movement of the Indian Plate. The arc was sutured to Asia at ca. 100 Ma. It was subsequently tilted northward when underplated by Indian continental crust during the early stages of India–Asia collision. Deep erosion of this tilted section provides a spectacular section through the whole arc sequence and offers a profound insight into the mechanisms of early stages of arc formation. Geochemical analysis and rare earth element modelling of basaltic sequences which date from the intraoceanic stages of arc development allow identification of three main magma source types in the mantle beneath the juvenile arc. The ‘E-type’ Kamila Amphibolites, with a MORB-type chemistry, form the intraoceanic basement to the arc. The ‘D-type’ Kamila Amphibolites are the earliest of the arc volcanic rocks. These were extracted from a primitive spinel-bearing mantle source, above a north-dipping subduction zone. The stratigraphically younger basalts of the Jaglot Group and Ghizar Formation of the Chalt Volcanic Group were derived from partial melting of a garnet-bearing source at greater depth. The Hunza Formation of the Chalt Volcanic Group contains the youngest mafic volcanic rocks of the intraoceanic arc. Although coeval with the Ghizar Formation of the Chalt Volcanic Group, they were generated by melting of a depleted, spinel-bearing mantle source rock and were erupted into a spatially and temporally restricted back-arc basin developed behind the volcanic front. The Chalt Volcanic Group was therefore formed from two different, adjacent, mantle source regions active at the same time. Results of REE modelling are consistent with models for intraoceanic arc formation in which the earliest volcanic rocks are derived from shallow level spinel-bearing peridotite, and later ones from a deeper garnet-bearing source. This is consistent with the melt region becoming deeper with time as subduction continues. A two-stage model is proposed for the back-arc basalts of the Hunza Formation in which a mantle source, depleted from a previous melting event, is underplated beneath the arc and later remelted during decompression as a consequence of extension and rifting of the arc

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the main rock units of the Chilas Complex, a series of lower crustal intrusions emplaced during initial rifting within the Mesozoic Kohistan (paleo)-island arc (NW Pakistan). Detailed field observations and petrological analysis, together with geochemical data, indicate that the two principal units, ultramafic rocks and gabbronorite sequences, originate from a common parental magma, but evolved along different mineral fractionation trends. Phase petrology and mineral trace element data indicate that the fractionation sequence of the ultramafic rocks is dominated by the crystallization of olivine and clinopyroxene prior to plagioclase, whereas plagioclase precedes clinopyroxene in the gabbronorites. Clinopyroxene in the ultramafic rocks (with Mg-number [Mg/(Fetot + Mg] up to 0·95) displays increasing Al2O3 with decreasing Mg-number. The light rare earth element depleted trace element pattern (CeN/GdN ∼0·5-0·3) of primitive clinopyroxenes displays no Eu anomaly. In contrast, clinopyroxenes from the gabbronorites contain plagioclase inclusions, and the trace element pattern shows pronounced negative anomalies for Sr, Pb and Eu. Trace element modeling indicates that in situ crystallization may account for major and trace element variations in the gabbronorite sequence, whereas the olivine-dominated ultramafic rocks show covariations between olivine Mg-number and Ni and Mn contents, pointing to the importance of crystal fractionation during their formation. A modeled parental liquid for the Chilas Complex is explained in terms of mantle- and slab-derived components, where the latter component accounts for 99% of the highly incompatible elements and between 30 and 80% of the middle rare earth elements. The geochemical characteristics of this component are similar to those of a low percentage melt or supercritical liquid derived from subducted mafic crust. However, elevated Pb/Ce ratios are best explained by additional involvement of hydrous fluids. In accordance with the crystallization sequence, the subsolidus metamorphic reactions indicate pressures of 0·5-0·7 GPa. Our data support a model of combined flux and decompression melting in the back-ar

Fractionation of sulfide phases controls the chalcophile metal budget of arc magmas: evidence from the Chilas complex, Kohistan arc, Pakistan

Some arc magmas lead to the formation of porphyry deposits in the relatively shallow upper crust (<5 km). Porphyry deposits are major sources of Cu and an important Au source but lack significant amounts of platinum group elements (PGE). Sulfide phases control the behavior of chalcophile elements and affect the potential to form ore deposits either by remaining in the mantle residue or by fractionating from arc magmas at lower crustal levels, although in detail the role of sulfide saturation in the lower crust remains poorly understood. Lower crustal cumulate rocks from the 85 Ma Chilas Complex of the Kohistan arc, Pakistan, provide insight into processes that occur at depth in arcs. Here we provide Cu, Ni, Au, and PGE concentrations and Os isotope ratios of the Chilas Complex in order to constrain the extent of sulfide saturation in the lower crust and the effect of sulfide saturation on the metal budget of evolved melts that ascend to the upper crust. The Chilas rock suite contains less than 0.17 wt % sulfides and low PGE concentrations. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) measurements of the sulfide inclusions in silicate minerals show enrichment in several chalcophile elements (up to 34 wt % Cu, 23 ppm Au, 245 ppm Pd, and 20 ppm Pt), whereas iridium group PGE (IPGE- Os, Ir, Ru) are mainly below detection limits. The metal content of the parental melt was modeled based on the elemental concentrations of the sulfides. The modeled parental arc magmas contain 70 to 140 ppm Cu, 0.2 to 1.5 ppb Au, and 1.2 to 8 ppb Pd, but low concentrations of IPGE, suggesting that IPGE were likely retained in the mantle source. Mass balance calculations show that segregation of a sulfide melt in the lower crust could further deplete the melt by more than 95% in Pd and Pt, 33 to 85% in Au, and 13 to 60% in Cu. Thus, magmas that ascend to the upper crust would contain very low concentrations of Au (< 0.2 ppb) and Pd (< 0.04 ppb), but they would retain sufficient concentration of Cu (~45–57 ppm) to form porphyry Cu deposits upon emplacement in the upper crust, as is commonly observed in arc settings

Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal and Sarangar complexes, which constitute the lower crust of the Mesozoic Kohistan paleo-island arc (Northern Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite with minor lenses of garnet hornblendite and granite, grading up-section to hornblende gabbronorite. The Sarangar complex is composed of metagabbro. The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, light rare earth element (LREE)-enriched REE patterns similar to those of island arc basalts. Together with the Jijal garnet granulite, they define negative covariations of LaN, YbN and (La/Sm)N with Eu* [Eu* = 2 × EuN/(SmN + GdN), where N indicates chondrite normalized], and positive covariations of (Yb/Gd)N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of the garnet granulites as plagioclase-garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration-melting of hornblende gabbronorite, provided that the amount of segregated or restitic garnet was low (30 km (equivalent to c. 1·0 GPa), together with the hot geotherms now postulated for lower island arc crust, should cause dehydration-melting of amphibole-bearing plutonic rocks generating dense garnet granulitic roots in island arcs. Dehydration-melting of hornblende-bearing plutonic rocks may, hence, be a common intracrustal chemical and physical differentiation process in island arcs and a natural consequence of their maturation, leading to the addition of granitic partial melts to the middle-upper arc crust and formation of dense, unstable garnet granulite roots in the lower arc crust. Addition of LREE-enriched granitic melts produced by this process to the middle-upper island arc crust may drive its basaltic composition toward that of andesite, affording a plausible solution to the ‘arc paradox' of formation of andesitic continental-like crust in island arc setting

Magma Transfer and Evolution in Channels within the Arc Crust: the Pyroxenitic Feeder Pipes of Sapat (Kohistan, Pakistan)

Our understanding of the mode of transfer and evolution of arc magmas in the lower arc crust is limited by the accessibility of arc roots, which are mainly documented by remote geophysical methods. At the same time, the fractionation processes of primitive parental melts defining a liquid line of descent from basalt to dacite are well defined by experimental petrology. However, the structural evidence for transfer of magmas evolving during their ascent remains basically uncharacterized. The Sapat Complex represents a lower crust segment of the exhumed Kohistan paleo-island arc and exposes kilometer-sized pyroxenite bodies that grew at the expense of host metagabbroic sill sequences. The largest of these pyroxenite bodies are mainly composed of wehrlite to olivine-clinopyroxenite, whereas the smaller bodies show a sequence of cumulative rocks, from ol-clinopyroxenite through various gabbros to tonalite. Inside the bodies, vertical magmatic and reactional structures indicate magma ascent accompanied by cumulate formation. Altogether, cumulates document the evolution of an initially primitive basaltic melt (at ∼7 kbar) that contained ≥5 wt % H2O. After cotectic olivine and clinopyroxene fractionation, the appearance of hornblende at the expense of clinopyroxene marks a stepping stone in the melt evolution. From this point, the appearance of orthopyroxene and hornblende at the expense of olivine drives the magma towards an andesitic composition, from which the crystallization of An-rich plagioclase and hornblende drives the melt to evolve further. During peritectic hornblende crystallization fluid-precipitated assemblages occur showing that the melts have reached water-saturation while they were crystallizing and percolating, thus degassing H2O-rich fluids. Structural observations, mineral and bulk-rock compositions, and calculated seismic P-wave velocities identify the ultramafic pipe-shaped bodies as magmatic conduits in which melt ascended from the mantle through the lower crust to feed upper crustal magma chambers and volcanic system

Geochemistry and provenance of the Lower Siwaliks from southwestern Kohat, western Himalayan Foreland Basin, NW Pakistan

Equivalent to the Lower Siwalik Group, the Late Miocene Chinji Formation in Pakistan consists of interbedded in-channel sandstone (SSt) and overbank mudstone (MSt) sequences. Twelve sandstone and sixteen mudstone samples from three different sections of the formation in southwestern Kohat, NW Pakistan were analyzed for major elements and selected trace elements. The Chinji sandstones are feldspathic and lithic arenites. They are mostly matrix-supported, moderately to well sorted, and contain angular to rounded framework grains. Authigenic carbonate makes up most of the matrix. The framework grains consist of abundant monocrystalline quartz, alkali feldspar, and lithic fragments with subordinate mica and trace to accessory amounts of heavy minerals including epidote, monazite, apatite, garnet, rutile, and brown hornblende. The lithic fragments consist of sedimentary, volcanic, and low-grade metamorphic rocks.The average concentration of Zr, Nb and Y, and the Ba/Sc and Ba/Co ratios in the studied samples are lower than the corresponding values for the upper continental crust (UCC) and Post-Archean Australian Shale (PAAS) indicating the presence of mafic phases in the source area(s). The high average Cr/Zr and Cr/V ratios of the investigated samples relative to UCC and PASS also support the presence of mafic lihtologies, possibly chromite and ultramafic rocks in the source region. The La/Sc and Th/Sc ratios of the Chingi samples are more like the UCC while the Th/Co and Cr/Th ratios suggest a major contribution from mafic rocks. The average percent differences of the Chinji samples from both the UCC and PAAS in terms of critical silicic to basic trace element ratios (Ba/Co, Ba/Sc, La/Co, La/Sc, Th/Co, Th/Sc, Zr/Cr, and Zr/Sc) suggest a mafic contribution of 23 to 47% (mudstone) and 56 to 69% (sandstone). The lower Th/U, Rb/Sr and Zr/Sc ratios in the studied samples than the corresponding values of the UCC and PAAS suggest negligible recycling for the sediments of the Chinji Formation. Petrographic point count data on the Chinji sandstone indicate sediment derivation from a dissected arc, suture belt, and recycled orogen corresponding to the Kohistan-Ladakh Arc, the Indus Suture Zone, and the Himalayan Tectonic units, respectively. The different source rocks identified on the basis of various petrographic and geochemical parameters occur as part of the mentioned tectonic domains

Comparisons between Tethyan Anorthosite-bearing Ophiolites and Archean Anorthosite-bearing Layered Intrusions: Implications for Archean Geodynamic Processes

Elucidating the petrogenesis and geodynamic setting(s) of anorthosites in Archean layered intrusions and Tethyan ophiolites has significant implications for crustal evolution and growth throughout Earth history. Archean anorthosite-bearing layered intrusions occur on every continent. Tethyan ophiolites occur in Europe, Africa, and Asia. In this contribution, the field, petrographic, petrological, and geochemical characteristics of 100 Tethyan anorthosite-bearing ophiolites and 155 Archean anorthosite-bearing layered intrusions are compared. Tethyan anorthosite-bearing ophiolites range from Devonian to Paleocene in age, are variably composite, contain anorthosites with highly calcic (An44-100) plagioclase and magmatic amphibole. These ophiolites formed predominantly at convergent plate margins, with some forming in mid-ocean ridge, continental rift, and mantle plume settings. The predominantly convergent plate margin tectonic setting of Tethyan anorthosite-bearing ophiolites is indicated by negative Nb and Ti anomalies and magmatic amphibole. Archean anorthosite-bearing layered intrusions are Eoarchean to Neoarchean in age, have megacrystic anorthosites with highly calcic (An20-100) plagioclase and magmatic amphibole and are interlayered with gabbros and leucogabbros and intrude pillow basalts. These Archean layered intrusions are interpreted to have predominantly formed at convergent plate margins, with the remainder forming in mantle plume, continental rift, oceanic plateau, post-orogenic, anorogenic, mid-ocean ridge, and passive continental margin settings. These layered intrusions predominantly crystallized from hydrous Ca- and Al-rich tholeiitic magmas. The field, petrographic and geochemical similarities between Archean and Tethyan anorthosites indicate that they were produced by similar geodynamic processes mainly in suprasubduction zone settings. We suggest that Archean anorthosite-bearing layered intrusions and spatially associated greenstone belts represent dismembered subduction-related Archean ophiolites