Growth of subcontinental lithosphere: evidence from repeated dike injections in the balmuccia lherzolite massif, Italian Alps

Abstract

The Balmuccia alpine lherzolite massif is a fragment of subcontinental lithospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists largely of lherzolite, with subordinate harzburgite and dunite, and an array of dike rocks formed in the mantle before crustal emplacement. Dike rocks include websterite and bronzitite of the Cr-diopside suite, spinel clinopyroxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbronorite of the late gabbro suite, and hornblendite of the hydrous vein suite. The dike rocks display consistent intrusive relationships with one another, such that Cr-diopside suite dikes are always older than dikes and veins of the Al-augite suite, followed by dikes of the late gabbro suite and veins of the hydrous vein Ž . suite. Phlogopite phl veinlets that formed during interaction with the adjacent crust are the youngest event. There are at least three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation series from spinel clinopyroxenite to websterite and feldspathic websterite, which crystallized from aluminous alkaline magmas at relatively high pressures. The late gabbro suite of dikes intruded at lower pressures, where plagioclase saturation occurred before significant mafic phase fractionation. Hornblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to the late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic compositions similar to those of the host lherzolite and within the range of compositions defined by ocean-island basalts. The Al-augite dikes and the hornblendite veins have Sr and Nd isotopic compositions similar to those of Cr-diopside suite lherzolite and websterite. There is overlap in the data between the Cr-diopside suite and the Al-augite and hydrous vein suites, with the exception that the Cr-diopside websterite dikes have more radiogenic Pb than any of the other samples. In Pb-Pb space as well, the late gabbro suite has the least radiogenic isotopic compositions, reflecting a change in magma source region during uplift. These data show that tectonic thinning of subcontinental lithospheric mantle during Ž . extension caused a change in the source regions of mantle-derived magmas from an ocean island basalt OIB -like lithosphere to the underlying MORB asthenosphere. They also demonstrate that the upper mantle acquires its heterogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, ) Corresponding author. Fax: q1-313-763-4690; E-mail: mukasa@umich.edu 0024-4937r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž . PII: S 0 0 2 4 -4 9 3 7 9 9 0 0 0 3 3 -X ( ) S. B. Mukasa, J.W. SherÕaisr Lithos 48 1999 287-316 288 dike-wall rock ionic exchange, redistribution of the lithosphe...

Partial melting of secondary pyroxenite at 1 and 1.5 GPa, and its role in upwelling heterogeneous mantle

“…If pyroxenite-derived melts are able to percolate and impregnate the adjacent peridotite, they are expected to form orthopyroxene-and clinopyroxene-enriched peridotites (e.g., Lambart et al 2012). Similar peridotites, refertilized by interaction with pyroxenite-derived melts, have been documented in various natural case studies (e.g., Pearson et al 1993;Rivalenti et al 1995;Varfalvy et al 1996; GV10 (this study) MM3 (Falloon et al 1999(Falloon et al , 2008 Robinson et al (1998) Page 19 of 24 _####_ Zanetti et al 1996;Mukasa and Shervais 1999;Bodinier et al 2004;Borghini et al 2013). Along the adiabatic upwelling of such heterogeneous mantle, this metasomatic process is likely to be continuous, as documented by experimental works on peridotiteeclogite reaction (Yaxley and Green 1998;Rosenthal et al 2014).…”

Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

“…The small size of the peridotite xenoliths makes it difficult to find evidence of melt infiltration. However, tectonically emplaced large peridotite massifs commonly show pyroxenite and/or mafic lenses, dikes and layers crystallized from basaltic melts (Mukasa & Shervais 1999; Takazawa et al . 2000).…”

Pyroxenite Layers in the Northern Apennines’ Upper Mantle (Italy)—Generation by Pyroxenite Melting and Melt Infiltration

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