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

The Balmuccia alpine Iherzolite massif is a fragment of subcontinental lith ospheric mantle emplaced into the lower crust 251 Ma ago during the final, extensional phase of the Hercynian orogeny. The Balmuccia massif consists l argely of Iherzolite, 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 clinopyr oxenite and spinel-poor websterite of the Al-augite suite, gabbro and gabbr onorite of the late gabbro suite, and hornblendite of the hydrous vein suit e. The dike rocks display consistent intrusive relationships with one anoth er, 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 vei ns of the hydrous vein suite. Phlogopite (phl) veinlets that formed during interaction with the adjacent crust are the youngest event. There are at le ast three generations of Cr-diopside suite dikes, as shown by crosscutting relations. Dikes of the Al-augite suite form a polybaric fractionation seri es from spinel clinopyroxenite to websterite and feldspathic websterite, wh ich crystallized from aluminous alkaline magmas at relatively high pressure s. The late: gabbro suite of dikes intruded at lower pressures, where plagi oclase saturation occurred before significant mafic phase fractionation. Ho rnblendite veins have distinct compositional and isotopic characteristics, which show that they are not related to either the Al-augite suite or to th e late gabbro dike suite. Cr-diopside suite dikes have Nd and Sr isotopic c ompositions similar to those of the host Iherzolite 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 Iherzolite and websterite. The late gabbro dikes have Nd and Sr isotopic compositions similar to mid-ocean ridge basalt (MORB) asth enosphere. Lead isotopic compositions for all of the samples fall in the pr esent-day MORE field on the Pb-208/Pb-204 vs. Pb-206/Pb-204 diagram but are displaced above this field on the Pb-207/Pb-204 vs. Pb-206/Pb-204 diagram. There is overlap in the data between the Cr-diopside suite and the Al-augi te and hydrous vein suites, with the exception that the Cr-diopside webster ite dikes have more radiogenic Pb than any of the other samples. In Pb-Pb s pace as well, the late gabbro suite has the least radiogenic isotopic compo sitions, reflecting a change in magma source region during uplift. These da ta show that tectonic thinning of subcontinental lithospheric mantle during extension caused a change in the source regions of mantle-derived magmas f rom an ocean island basalt (OIB)-like lithosphere to the underlying MORE as thenosphere. They also demonstrate that the upper mantle acquires its heter ogeneous isotopic character through several different processes, including in situ radiogenic growth, addition of asthenospheric melts, dike-wall rock ionic exchange, redistribution of the lithospheric dike and vein materials by melting, and in the late stages of emplacement, assimilation of crustal materials. (C) 1999 Elsevier Science B.V. All rights reserved.