John Malpas

Model for the origin of the Troodos massif, Cyprus, and other mideast ophiolites

Any comprehensive model for the origin of the Troodos complex and other Mideast ophiolite complexes must explain the geochemical evidence for subduction-zone involvement, the thin inferred oceanic crust, the extensional environment indicated by sheeted dikes, the existence of fault zones perpendicular to the inferred spreading axes, and the discontinuous nature of ophiolite exposures around the Arabian block. The Andaman Sea region of the Indian Ocean may provide an actualistic model for the origin of these ophiolites. There, spreading takes place in short segments above a subduction zone in the region of active spreading. The Andaman Sea model substantially accounts for the known geologic and geophysical data from the circum-Arabian ophiolite belt, and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt. The model also predicts that Andaman Sea spreading-center magmas may be similar in composition to those found in the Mideast ophiolites.

The origin of oceanic plagiogranites from the karmoy ophiolite, western Norway

Both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the Karmoy ophiolite of western Norway. The plutonic zone of this ophiolite can be subdivided into three complexes; the East-Karmoy Igneous Complex, the Visnes High Level Complex and the Veavagen Igneous Complex and plagiogranitic rocks are well developed in the first two of these.Within the East-Karmoy Igneous Complex, plagiogranites are associated with high temperature, pre-basic dyke, shear zones. Rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite (hydrated diabase) assuming a starting material consisting of 40% hornblende and 60% plagioclase and that batch melting occurred within the stability field of hornblende.In comparison, plagiogranite occurs in a number of bodies in the upper part of the Visnes High Level Complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes, microgabbros and magnetite gabbros, and a floor assemblage of layered and non-layered gabbros. The R.E.E. modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari-textured gabbros, although the distribution of highly hygromagmatophile elements such as K, Rb, Ba, etc. cannot be explained satisfactorily by this model alone. Depletion in these elements appears to be an autometasomatic effect.

Geochemical and geochronological constraints on the origin and emplacement of the Yarlung Zangbo ophiolites, Southern Tibet

Abstract The Indus-Yarlung Zangbo suture zone in southern Tibet marks the Eocene collision of the Indian continent and the Lhasa Block of Eurasia. It is characterized, particularly in its central portion, by an east-west belt of ophiolitic and related oceanic volcanic and sedimentary rocks that form a number of structurally juxtaposed geological terranes. Although tectonically disrupted in many places, almost complete ophiolite sequences exist at Luobusa and Zedong in the east and near Xigaze in the west. In Luobusa, the ophiolite sequence is thrust over the Tertiary molasse deposits of the Luobusa Formation or onto plutonic rocks of the Gangdese batholith. A mantle sequence dominates the ophiolite massif and consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with abundant podiform chromitites enveloped by dunite. The Luobusa ophiolite formed the basement to an intra-oceanic volcanic arc, the Zedong terrane, which developed between the Mid-Jurassic and Mid-Cretaceous. Farther to the west, complete ophiolite sequences exist at Dazhuqu and near Xigaze. These ophiolites have suprasubduction zone geochemical signatures but there is no apparent development of a volcanic arc. Sensitive high-resolution ion microprobe U-Pb zircon analyses yield an age of 126 Ma for the crystallization of a quartz diorite from the Dazhuqu massif. Amphibolites that occur as large blocks in mélanges at the base of the ophiolites are considered to be remnants of dynamothermal metamorphic soles produced early in the ophiolite obduction process. Ar/Ar geochronology on amphibole and biotite separates from these rocks yields ages of 80 and 90 Ma, respectively, for this event, which is considered to have occurred as the Indian continental margin entered the intra-oceanic subduction zone. Continued northward subduction of the remaining portion of the Neo-Tethyan ocean floor beneath the southern margin of Eurasia produced the Gangdese continental arc on the southern margin of the Lhasa Block and led to the final closure of the ocean with the collision of India and Eurasia in the Eocene.

Ultra-high pressure minerals in the Luobusa Ophiolite, Tibet, and their tectonic implications

Abstract Numerous ultra-high-pressure minerals have been recovered from podiform chromities in the Luobusa ophiolite, Tibet. Recovered minerals include diamond, moissanite, Fe-silicides, wüstite, Ni-Fe-Cr-C alloys, PGE alloys and octahedral Mg-Fe silicates. These are accompanied by a variety of native elements, including Si, Fe, Ni, Cr and graphite. All of the minerals were hand-picked from heavy-mineral separates of the chromitites and care was taken to prevent natural or anthropogenic contamination of the samples. Many of the minerals and alloys are either enclosed in, or attached to, chromite grains, leaving no doubt as to their provenance. The ophiolite formed originally at a mid-ocean ridge (MOR) spreading centre at 177±33 Ma, and was later modified by suprasubduction zone magmatism at about 126 Ma. The chromitites were formed in the suprasubduction zone environment from boninitic melts reacting with the host peridotites. The UHP minerals are believed to have been transported from the lower mantle by a plume and incorporated in the ophiolite during seafloor spreading at 176 Ma. Blocks of the mantle containing the UHP minerals were presumably picked up by the later boninitic melts, transported to shallow depth and incorporated in the chromitites during crystallization.

Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high-temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures >450–600°C resulted in entrapment of brine-rich aqueous fluids with salinities of 36–61 wt % NaCl equivalent. Homogenization of the brine inclusions by haute dissolution, the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz-hosted, high-salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep-seated, high-temperature portion of the hydrothermal system. In localized areas, the high-temperature brines (NaCl±KCl±CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low-temperature, low-salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high-temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures >200–400°C. Hydration reactions under greenschist facies conditions, or limited mixing with brine-rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low-salinity inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid-ocean ridge spreading centers. The low-temperature fracture networks may represent an extensive deep-seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.

A review of the petrology of harzburgites at Hess Deep and Garrett Deep: implications for mantle processes beneath segments of the East Pacific Rise

Abstract In recent years a unique set of samples of uppermost mantle at the mantle-crust transition zone have been collected from two different environments along the fast-spreading East Pacific Rise (EPR): a ‘normal’ spreading segment (represented by samples from Hess Deep) and the end of a spreading segment where the EPR meets the Garrett transform fault (represented by samples from Garrett Deep). A review of the petrology of harzburgites from the two sites demonstrates that these rocks were produced by partial melting of adiabatically upwelling mantle and, subsequently, at the top of the mantle, they were impregnated by reactive and crystallizing mid-ocean ridge basaltic (MORB) melts. Despite this similar history, non-impregnated harzburgites at Garrett Deep have a more fertile spinel chemistry than those at Hess Deep, which is consistent with reduced partial melting of shallow mantle as a transform fault is approached — the ‘transform fault effect’. The extent of reaction between melt and harzburgite during the impregnation event suggests that melt arrived in the uppermost mantle in a highly reactive state because along the adiabatic path it had been highly channelled in spatially restricted conduits. This implies that mantle upwelling below the EPR was, and presumably still is, dominantly two dimensional (sheet-like). Within this framework, the chemical evolution of MORB melt below fast-spreading ridges will be significantly affected by melt-periodotite reaction only when melt reaches the uppermost mantle and mantle-crust transition zone, where along-axis transport of melt may also be important. Although the harzburgites from Hess Deep and Garrett Deep formed and evolved beneath different parts of different first-order segments of the EPR, the petrology of these rocks presents the best analogue available for defining real variations in mantle processes along a single first-order ridge segment in a fast-spreading environment.