Garnet-bearing Pyroxenites Research Articles

Lower crustal evolution under central Arizona: Sr, Nd and Pb isotopic and geochemical evidence from the mafic xenoliths of Camp Creek

Oligocene potassic volcanism at the Camp Creek area, Arizona has sampled the lower crust by bringing mafic granulite, amphibolite and garnet-bearing pyroxenite (eclogite) xenoliths to the surface. Xenolith geobarometry records metamorphic pressures of approximately 10 kbar and temperatures of 650–900°C. Whole-rock Sr and Nd isotopic compositions correlate roughly with xenolith metamorphic mineral assemblages: higher temperature eclogites have 87Sr/ 86Sr ranging from 0.7066 to 0.7080 and ε Nd of −2 to −5.8 while lower temperature amphibolites have 87Sr/ 86Sr of 0.7074 to 0.7082 and ε Nd of −6 to −9. Sr and Nd isotopic compositions of mineral separates reflect, in some xenoliths, intermineral diffusion up to the time of latite eruption and limited exchange with the host latite. Pb isotopic compositions of the xenoliths show little mineralogical dependence and overlap the field for normal MORB. These data suggest that lower-crustal metamorphism has significantly affected the Sr and Nd isotope evolution of the eclogites but has had little effect on their Pb isotope systematics. For xenoliths closer to igneous bulk compositions (i.e. not enriched in metamorphic garnet and pyroxene), Nd depleted mantle model ages range from 1.2 to 1.9 Ga, in agreement with the Pb Pb array “age” of 1.1 ± 0.6Ga. This is interpreted as dating the separation of the lower crust from the mantle during the Proterozoic crust-building event in this area. Similar Nd model ages for nearby Cenozoic and Mesozoic granitoids suggest that the type of mafic lower crust represented by the Camp Creek xenoliths may be a suitable source for younger plutonics in the area.

Mapping present-day geochemical variations across the society hotspot

The presence of variable amounts of amphibole ± phlogopite in a garnet websterite and a garnet clinopyroxenite from the Beni Bousera peridotite massif provides evidence for post-formation metasomatism. Textural observations associated with major- and trace-element mineral compositions allowed us to distinguish two metasomatic episodes, which occurred at different stages of the Beni Bousera massif evolution. The garnet websterite has recorded interaction with LREE-rich silicate melts before the uplift of the massif. Amphibole/clinopyroxene and amphibole/garnet trace-element ratios closely approach partition coefficient values, indicating that chemical equilibrium was attained between amphibole and pyroxenite matrix minerals. The geochemical signatures of the putative alkaline interacting melts are similar to those of recent basaltic magmas erupted in Morocco, suggesting a common peridotite mantle source. In contrast, amphibole from the garnet clinopyroxenite is in chemical disequilibrium with the pyroxenite matrix minerals. In this clinopyroxenite the crystallization of amphibole and plagioclase occurred at lower T (and P) conditions, most probably during the ascent of the Beni Bousera massif and its emplacement into the crust. The melt responsible for this later metasomatic episode was LREE-depleted and HREE-enriched, suggesting that it resulted from decompression melting of a garnet-bearing source (with garnet as a melting phase), similar to the garnet-bearing pyroxenites outcropping in the Beni Bousera massif.

Petrology of volcanic rocks from Kaula Island, Hawaii

A compositionally diverse suite of volcanic rocks, including tholeiites, phonolites, basanites and nephelinites, occurs as accidental blocks in the palagonitic tuff of Kaula Island. The Kaula phonolites are the only documented phonolites from the Hawaiian Ridge. Among the accidental blocks, only the phonolites and a plagioclase basanite were amenable to K-Ar age dating. They yielded ages of 4.0–4.2 Ma and 1.8±0.2 Ma, respectively. Crystal fractionation modeling of major and trace element data indicates that the phonolites could be derived from a plagioclase basanite by subtraction of 27% clinopyroxene, 21% plagioclase, 16% anorthoclase, 14% olivine, 4% titanomagnetite and 1% apatite, leaving a 16% derivative liquid. The nephelinites contain the same phenocryst, xenocryst and xenolith assemblages as the tuff. Thus, they are probably comagmatic. The strong chemical similarity of the Kaula nephelinites and basanites to those from the post-erosional stage Honolulu Group on Oahu, the presence of garnet-bearing pyroxenites in the Kaula nephelinites (which previously, had only been reported in the Honolulu volcanic rocks) and the similar age of the Kaula basanite to post-erosional lavas from nearby volcanoes are compelling evidence that the Kaula basanites and nephelinites were formed during a “post-erosional” stage of volcanism.

A petrologic model for the constitution of the upper mantle and crust of the Koolau shield, Oahu, Hawaii, and Hawaiian magmatism

A petrological model for the uppermost upper mantle and crust under the Koolau shield to a depth of about 60 km has been derived on the basis of petrology of the upper mantle and crustal xenoliths in nephelinites of the Honolulu Volcanic Series. Three main xenolith suites exist in the Koolau shield: dunites, spinel lherzolites, and garnet-bearing pyroxenites. On the basis of mineralogy, it is inferred that the dunites represent cumulates in shallow crustal tholeiitic magma chambers, the spinel lherzolites form a thick (∼ 40 km) layer in the upper mantle, and the garnet pyroxenite suite occurs as veins and stringers in the spinel lherzolites at about 60 km depth. The eruption sequence in a Hawaiian volcano, i.e., tholeiite → transitional basalt → alkali basalt, is generated by partial melting of a volatile-bearing garnet-lherzolite part of a lithospheric plate as it rides over a hot spot. If the tholeiite, transitional, and alkali basalts of Hawaiian volcanoes are generated at the same depth, then the observed sequence of lavas requires replenishment of the source area with volatiles and gradual decrease of the degree of partial melting with time. Post-erosional olivine nephelinites are produced from isotopically distinct, deeper source area, which may be the asthenosphere.