ALBERT

Description: New insights into the origin of high-Mg andesites are inferred from the mineral chemistry and U–Pb geochronology of Tertiary amphibole-rich ultramafic intrusive rocks (hornblendites) and included clinopyroxene-bearing dunitic clots from the southern Adamello batholith (Central Alps). The hornblendites consist mostly of amphibole grains with brown cores (Ti-pargasite) that grade through brownish-green (Mg-hornblende) to light green (edenite) rims. Brown amphibole contains olivine (Fo = 85–87 mol %) and clinopyroxene inclusions with irregular boundaries indicating disequilibrium with the host amphibole. The ultramafic clots are interpreted to represent fragments of older cumulates dismembered by the injection of the amphibole-forming melts, thereby providing evidence for a melt–rock reaction process. Amphibole from the hornblendites shows a marked trace element zoning. From the brown core outward to the brownish-green portion of a single crystal, a significant enrichment is observed in light rare earth elements, Th and U, coupled with a decrease in Ti and heavy rare earth elements. The melt in equilibrium with the brownish-green amphibole has an adakitic trace element signature (e.g. high La N /Yb N and Sr/Y). Based on amphibole/liquid partition coefficients, a fractional crystallization process driven by amphibole could explain most of these chemical variations. However, the outward increase of highly compatible elements in amphibole (e.g. Mg, Ni, Co, and Zn) argues against closed-system fractional crystallization. The assimilation of olivine is considered the most efficient mechanism to supply or buffer the highly compatible elements in the evolving system during amphibole crystallization. In situ U–Pb zircon geochronology of hornblendites and associated amphibole gabbros reveals the occurrence of inherited cores, thereby providing evidence for assimilation of crustal material. We propose that a differentiation process controlled by amphibole crystallization and assimilation of slightly older ultramafic cumulates may produce melts rich in SiO 2 and MgO with adakitic trace element signatures.

Description: New insights into the role of amphibole in arc magma petrogenesis are provided by the mineral chemistry and U–Pb geochronology of Cretaceous amphibole-rich mafic rocks and associated granitoids from Shikanoshima Island (Kyushu, Japan). In the northeastern part of Shikanoshima Island a relatively large body (about 600 m in length) of amphibole-rich mafic rocks is found within granodiorite host-rocks. The core of the mafic body consists of amphibole-rich gabbrodiorite with a porphyritic texture. Towards the host granodiorite the porphyritic texture is progressively lost and a band of relatively homogeneous medium- to fine-grained mafic rock marks the boundary with the granitoid rocks. The amphibole-rich porphyritic gabbrodiorite consists of large amphibole grains (up to 60 vol. %) characterized by brown cores, occasional inclusions of clinopyroxene, and green rims. These large amphibole grains are dispersed in a fine-grained matrix consisting of green amphibole, clinopyroxene and plagioclase. Literature whole-rock data on the mafic rocks from Shikanoshima Island suggest that they are the intrusive counterparts of high-Mg andesite (HMA). Major and trace element mineral compositions reveal a marked chemical contrast between the brown amphibole (and its inclusions) and the matrix minerals, suggesting that they are not on the same liquid line of descent. The brown amphibole and its clinopyroxene inclusions were inherited from amphibole-rich ultramafic intrusive crustal rocks (e.g. hornblendites) crystallized from a melt with a chemical composition close to that of continental arc basalts. U–Pb geochronological data suggest that the xenocrystic material is about 20 Myr older than the matrix minerals. The matrix mineral crystallized from a parental liquid similar to sanukite-type HMA and with a trace element signature characterized by strong enrichment in elements with high crustal affinity and depletion in heavy rare earth elements. Green amphibole is a common mineral in all the studied lithologies; this allowed us to monitor the compositional variations in the liquid from which it crystallized moving from the core of the mafic complex to the host granodiorite. The data reveal that the interstitial melt had interacted with a melt enriched in elements with a high crustal affinity that, given the close association in the field, is inferred to be the host granitoid. These results favour an origin for sanukite-type HMA not from primary mantle melts but from mantle melts that have been affected by crustal processes and have been contaminated by crustal material. The major and trace element composition of the brown amphibole from the Shikanoshima Island mafic rocks is compared with that of brown amphibole from other amphibolite-rich intrusive rocks in orogenic settings worldwide (Alpine chain and Ross Orogen). The observed similarities suggest that the amphibole-rich mafic rocks are the expression of a magmatic process with a common geochemical affinity that is independent of the age and local geodynamic setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks could be a common feature of all collisional systems and thus represent a ‘hidden’ amphibole reservoir in the arc crust. We show that amphibole plays a major role in the petrogenesis of sanukite-type HMA but is also expected to play a major role in the differentiation of many other arc magmas.