Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation

A review of criteria for the identification of magmatic and tectonic foliations in granitoids

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

Orogenic, ophiolitic and abyssal peridotites

Recently developed ideas of global tectonics haye provided a new framework within which to consider the origin of alpine-type peridotites In plate theory, compressional zones associated with island arcs are considered to represent plate boundaries where oceanic lithosphere is subducted The subduction zones are characterized by lithospheric underthrusting, andesitic volcanoes, and deep seismic activity that generally dips under the continental edge (the Benioff zone) The presence of large oceanic-mantle crustal slabs thrust over or into continental edges contemporaneously with blueschist metamorphism in New Caledonia and New Guinea establishes an important variant of plate tectonics in the zones of compression The ‘obduction’ zones are characterized by a complete lack of volcanic activity and by high-pressure metamorphism During formation, they can be represented by shallow seismic zones dipping oceanward The common association of peridotites and blueschists in these orogenic belts may result from the initial stage of compressional impact (or orogeny) between an oceanic and a continental lithospheric plate Disturbed zones combined with a lack of high-temperature contacts at boundaries between cold mantle-peridotite slabs and trench sediments provide geologic evidence of emplacement by obduction (tectonic overriding) Internal subsolidus plastic deformation of these peridotites can be attributed to deep-seated strain within the upper mantle during spreading Serpentinites represent alteration developed during tectonic emplacement into wet sediments of the continental plate, which produces a less dense and plastic envelope that facilitates further tectonic movement in these compressional zones Recognition of these peridotite-serpentinite-blueschist belts within exhumed subduction or obduction zones will allow delineation of ancient compressional impacts between moving lithospheric plates

Plate tectonic emplacement of upper mantle peridotites along continental edges

We compared the magnetic fabric with the macroscopic surface lineation direction (previously assumed to be parallel with magma flow direction) for 35 dikes of the Koolau complex, Oahu. Anisotropy of magnetic susceptibility (AMS) was determined for 530 samples from 59 dikes (including a sill and a pluglike intrusive body) and two filled lava tubes, and statistically significant AMS clusters were found in all but five dikes. For the 35 lineated dikes, the mean maximum AMS direction (χ1) coincides with the macroscopic lineations to within 25° in 18 and to within 15° in eight. The AMS ellipsoid shape in about half of the samples is prolate (cigar shaped) and in about half is oblate (pancake shaped). We interpret the AMS to be determined by shape anisotropy of ferromagnetic grains. The mean total anisotropy is 2.2%, and total anisotropy (H) ranges from 0.24% to as high as 14.12%. The principal AMS axes for samples from a dike are tightly grouped on equal-area plots, and for most of the dikes the minimum AMS axes plot near to the pole of the dike. Our AMS measurements enable us to infer absolute magma flow directions for half the dikes studied; the maximum AMS axes of samples from one side of the dike form a tight cluster on an equal-area plot, and those from the other side of the dike form another tight cluster. The two clusters are close together (separated by 10–30°) for narrow dikes (<100 cm wide), and are farther apart (30–60°) for wide dikes and are symmetrically disposed on either side of the plane of the dike. We infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient, and hence indicate the flow azimuth, i.e., the absolute magma flow direction. Nearly half of the dikes measured (25) gave paired AMS plots; in 12 the magma flowed up at low oblique angles 5–30° from the horizontal, in five it flowed down at 5–20° in four it flowed horizontally (<5°), in three it flowed up at a steep angle (50–75°), and in one it flowed down at a steep angle. The magma flow in general was rarely vertical or horizontal.

Magma flow directions in dikes of the Koolau Complex, Oahu, determined from magnetic fabric studies

Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie

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Origin of ultramafic rocks, their tectonic setting and history: A contribution to the discussion of the paper “the origin of ultramafic and ultrabasic rocks” by P.J. Wyllie