Trace element chemistry and U–Pb dating of zircons from oceanic gabbros and their relationship with whole rock composition (Lanzo, Italian Alps)

Abstract

The U-Pb ages and the trace element content of zircon U-Pb along with major and trace element whole rock data on gabbroic dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean-continent transition zones. Three Fe-Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement mechanism of gabbroic rocks in oceancontinent transition zones evolves from in situ crystallization to stratified crystallization with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history is probably related to the thermal evolution of the underlying mantle lithosphere.

The Betic Ophiolites and the Mesozoic Evolution of the Western Tethys

“…These ages have been confirmed by the more recent dating of samples from Cóbdar and Algarrobo outcrops, shown in Figures 6 and 7 and Table 1 of this paper, whereas the SHRIMP U-Pb ages of gabbroic rocks from the Alps, Corsica and Apennines (with red color in Figure 8) yield comparatively younger ages, beginning at around 165 Ma. These Middle-Upper Jurassic ages for igneous zircons in gabbros and plagiogranites from the Alpine-Apennine Ophiolites (Figure 8) are also commonly accepted by other authors (Borsi et al [67]; Costa and Caby [68]; Rubatto and Scambelluri [69]; Kaczmarek et al [70]; Manatschal and Müntener [28]) and conform within the chronological interval 165-140 Ma. Thus, according to the SHRIMP zircon dating shown in Figures 6-8, and to the data compilation in Manatschal and Müntener [28], we can conclude that the break-up of Pangea and the development of the Western Tethys oceanic-floor began at the westernmost sector, from which the Betic Ophiolites derive (Betic Tethys) about 20 Ma earlier than in the northeastern extension (Ligurian and Alpine Tethys), from which the Alpine-Apennine Ophiolites originated.…”

Re-evaluating the geochronology of the Permian Tarim magmatic province: implications for temporal evolution of magmatism

“…Zircon crystallizes in a differentiated gabbroic environment and can be used to represent the crystallization age of host rock (e.g. Kaczmarek et al 2008). However, zircons in a mafic environment usually experience a complex history, which makes it necessary to identify zircon origins and group populations by variable approaches such as zircon chemistry and morphology (e.g.…”

U–Pb and Sm–Nd geochronology of the ophiolites from the SE Turkey: implications for the Neotethyan evolution

“…U-Pb crystallization ages of the oceanic magmatism (gabbro) reported from the Western and Central Alps ranges from 164 to 158 Ma (Kaczmarek et al, 2008;Rubatto & Gebauer, 1998;Rubatto & Scambelluri, 2003). But the youngest oceanic magmatism was reported as 93 Ma by Liati et al (2003) in the Central Alps.…”

“…Nowadays, the most powerful tool for the isotopic dating applicable to ophiolites includes U-Pb dating of zircons in evolved rocks such as gabbros, diorites or plagiogranites (e.g., Dilek & Thy, 2009;Karaoğlan, Parlak, Kloetzli, Thöni, & Koller, 2013;Liati, Gebauer, & Fanning, 2004;Tilton, Hopson, & Wright, 1981;Warren, Parrish, Waters, & Searle, 2005). Many studies show that there have been a great number of cases from Alpine-Himalayan orogenic belt, where co-genetic zircon is reported to be present in magmatic (gabbro and plagiogranite) and metamorphic (eclogite and amphibolite) rocks associated with ophiolites (Dilek & Thy, 2006, 2009Dilek et al, 2008;Gray et al, 2004;Gebauer, 1990Gebauer, , 1999Kaczmarek, Müntener, & Rubatto, 2008;Karaoğlan, Parlak, Kloetzli, Thöni, et al, 2013;Koglin, 2008;Konstantinou, Wirth, & Vervoort, 2007;Lapen, Medaris, Brandon, Johnson, & Beard, 2007;Liati, 2005;Liati, Gebauer, & Fanning, 2003Miller, Mundil, Thöni, & Konzett, 2005;Mukasa & Ludden, 1987;Pedersen, Searle, Carter, & Bandopadhyay, 2010;Pedersen, Searle, & Corfield, 2001;Rubatto & Gebauer, 1998;Rubatto, Gebauer, & Compagnoni, 1999;Rubatto & Scambelluri, 2003;Tilton et al, 1981;Warren et al, 2005) (Figure 1). …”