ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Zircons from a suite of basic to acidic calcalkaline igneous rocks from the southern Adamello batholith (S AB), Southern Alps, N Italy, display complex U−Pb isotopic patterns which are mainly due to the presence of variable amounts of isotopically heterogeneous, inherited radiogenic Pb, and to minor postmagmatic loss of Pb. Inherited Pb is mainly composed of 1) a ∼ 1100 Ma Pb component located in zircons devoid of visible cores and 2) a ∼ 450 Ma component associated with conspicuous bubble-rich turbid cores. In concordia representation the data points conform to lower intercept ages of ∼ 40 Ma. A linear fit of three samples devoid of visible cores from the granodiorite defines an intrusion age of ∼ 39.3 Ma. U−Pb systematics of zircon (in particular U content) and crystal morphology are clearly related. Zircons of type G1, which form relatively late in the zircon crystallization sequence, consistently show the highest U contents in each zircon population. These late zircons, however, are not devoid of inherited radiogenic lead. In a population from a granodiorite, cores are randomly distributed throughout the morphological spectrum. The presence of old inherited zircon components in all investigated samples furnishes proof for involvement of crustal material in the genesis of the S AB rocks. Samples characterized by crustal ɛNd and ɛSr values usually show enhanced zircon inheritance. Inheritance varies with differentiation and reaches a maximum for intermediate to acidic members; these compositions show the highest Zr saturation temperatures calculated for the rock spectrum studied. Textural relations between zircon and major phases indicate that the magmas of the leucocratic rocks were saturated with Zr at an early stage of crystallization. On the other hand, Zr solubilities and textural relations consistently show, that melts of basic to intermediate rocks were not saturated with Zr. Extension of the Zr solubility model to mineral/melt mixtures of tonalitic bulk composition demonstrates that Zr solubility in the residual melt is drastically reduced by crystallization of plagioclase and amphibole. Survival of xenocrystic zircons in the mafic to intermediate rocks of the S AB can best be explained in terms of dissolution kinetics. Since temperature and H2O content of these Zr-undersaturated melts were favourable for relatively rapid zircon dissolution, inherited zircons (in particular trace-element rich unstable cores) cannot have been exposed to such conditions over extended time periods. Therefore, the tonalitic or more basic magmas of the S AB cannot have been derived from crustal sources by slow processes such as burial metamorphism.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00310684
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