Geochemistry and Crystal-Chemistry of Mantle Amphiboles from the Carphato-Pannonian Region and their
Petrological Implications

A. Zanetti Dipartimento Scienze della Terra, Università di Pavia, I-27100 Pavia, Italy

R. Vannucci Dipartimento Scienze della Terra, Università di Pavia, I-27100 Pavia, Italy

L. Ottolini C.N.R. - Centro di Studio per la Cristallochimica e la Cristallografia, I-27100, Pavia, Italy

R. Oberti C.N.R. - Centro di Studio per la Cristallochimica e la Cristallografia, I-27100, Pavia, Italy

Amphibole megacrysts occur within lavas and pyroclastic deposits of the late Tertiary/Quaternary alkaline volcanism in the Pannonian and Transylvanian Basins. Lava flows and pyroclastic deposits also carry peridotitic mantle xenoliths (spinel-facies assemblages), which contain selvages or veins of pure amphibole and amphibole-bearing pyroxenites. Thirteen megacrysts and seven vein amphiboles were selected from samples belonging to six different volcanic centres with the aim to provide a better understanding of genetic relationships between vein amphiboles and megacrysts and to determine whether trace-element variations and crystal-chemical constraints can be applied to the interpretation of the processes which affected the subcontinental mantle beneath the Carphato-Pannonian Region (CPR).

Amphiboles crystals were refined to R values of ~2% using single-crystal MoKa X-ray data, and were subsequently analysed by means of electron- and ion-microprobe. In particular, the combination of crystallographic information and electron-microprobe data permitted an accurate estimate of the Fe3+/FeT value and of the hydrogen content.

Vein amphiboles have pargasitic to Mg-hastingsitic compositions. Megacrysts show higher Ti, Fe2+, K and [4]Al, but lower Fe3+/FeT and M(4)Mg contents, and can be classified as kaersutites. The higher [6]Fe2+, AK and [4]Al content in megacrysts determine homogeneous enlargements of the octahedral strip, of the A site and of the tetrahedral double-chain, respectively; as a result, a significant increase in the unit-cell volume is observed (the unit-cell volume is typically 904.9-909.9 Å and 909.0-915.7 Å in vein amphiboles and megacrysts, respectively). Moreover, the negative correlation between Fe3+/FeT and Ti suggests a progressive decrease in fO2 from vein amphiboles to megacrysts; such a variation could be related to lower XH2O in the melts, as suggested by the lower hydrogen content found in megacrysts.

As for trace elements, megacrysts show LREE-enriched, convex-upward REE patterns (normalised to C1 chondrite), with the maximum at Nd or Sm (LaN/SmN = 0.69-0.93; LaN/YbN = 2.33-4.80). The absolute concentrations of most incompatible elements (REE, Zr, Nb, Sr, Ba, Rb, Y), as well as the LaN/SmN and LaN/YbN values increase, whereas those of the less incompatible elements such as Sc and V decrease with decreasing Mg* values; these behaviours are consistent with a progressive trend of magmatic fractionation. As a whole, trace-element data indicate that megacrysts, which are characterised by large Ba, Sr and HFSE anomalies, were in equilibrium with LREE-enriched magmas similar to the host alkali basalts.

Vein amphiboles have more variable compositions. Most vein amphiboles have chondrite-normalised REE patterns similar to those of megacrysts, with LREE only slightly less fractionated (LaN/SmN = 1.04-1.19; LaN/YbN = 3.04-3.24) and Ba, Sr and HFSE anomalies. More rarely, vein amphiboles show smoother patterns, with a steep and regular fractionation from LREE to HREE (LaN/SmN and LaN/YbN up to 1.86 and 7.45, respectively) and without significant Ba, Sr and HFSE anomalies, thus approaching the composition of alkaline magmas. Trace-element concentrations of vein amphiboles define a fractionation trend, which, with the exception of alkaline elements (Rb, Ba and Sr), is opposite to that shown by megacrysts, i.e. most incompatible elements decrease, whereas less incompatible elements increase with decreasing Mg* values.

As a whole, compositional differences suggest that vein amphiboles crystallised at higher P-fO2-XH2O conditions than megacrysts. Vein amphiboles formed from fluids ascending from a sub-lithospheric mantle source, possibly the same that generated host alkali basalts; amphiboles with and without LILE and HFSE fractionation are believed to
be crystallised under closed- and open-system conditions, respectively. Megacrysts formed as liquidus phases of progressively differentiated basaltic magmas. The geochemistry and crystal-chemistry of amphiboles crystallised under the CPR subcontinental-mantle conditions thus indicate a strong relationships between volcanic and deep-seated processes; in particular, the enrichment of the lithospheric mantle seems to be related to the upward migration of
primitive alkaline liquids similar to the host alkali basalts.