Petrology and geochemistry of microgranular mafic enclaves from the Vedrette di Ries plutonic complex (Eastern Alps)

The microgranular mafic enclaves included in granitoid bodies are of high petrological interest, inasmuch as they represent records of previous magmatic activity and/or episodes of mafic magma input into evolving granitoid systems. In this paper we report major, trace clement and preliminary Sr isotopic data for a series of mafic inclusions coming from the calcalkaline intrusive complex of Vedrette di Ries (VdR), an Alpine plutonic body cropping out in the Eastern Alps and composed of prevailing granodiorites and tonalites with minor granites and diorites. The analyzed enclaves have a fine grained allotriomorphic to subidiomorphic texture and consist of variable amounts of zoned plagioclase, amphibole (mainly hormblende with minor actinolite), biotite and quartz. Zircon, allanite, apatite and Fe-Ti oxides are the main accessory phases. Secondary chlorite and muscovite are sometimes present. Chemical compositions range from basic to intermediate and are characterized by higher abundance of ferromagnesian elements, K20, Rb and HREE with respect to the VdR intrusive rocks with a similar Differentiation Index. Sr isotopic ratios range from 0.7096 to 0.7109 and are similar or close to those of the enclosing rocks. REE patterns are variably fractionated with LaN/YbN from lower to much higher than unity and are characterized by negative anomalies of Eu. One sample has flat REE pattern with a positive Eu spike. These patterns are strikingly different from those observed not only in the host intrusive but also in typical worldwide basic to intermediate calcakaline rocks; overall, the compositional characteristics of the VdR inclusions appear to be heavily mineral-dominated. This suggests that the studied inclusions do not represent liquids but rather reflect heterogeneous concentration of main and accessory phases. The high abundance of ferromagnesian elements in many of the samples and especially the occurrence of REE patterns with positively fractionated LREE (LaN/SmN < 1) and significant negative Eu anomalies suggest that hornblende was the phase which more heavily contributed to the composition of the studied inclusions. The presence of one sample with poorly fractionated REE pattern and a positive Eu anomaly points to a role of plagioclase concentration in some inclusions. The variable LREE abundance, the positive correlation between Th and LREE, however, also points to heterogeneous concentration of allanite. Textural evidence tends to exclude the possibility that the microgranular mafic enclaves from VdR derive from accumulation by gravitational settling of mineral phases cristallized from the evolving granitoid magma. More probably, they represent previously intruded calkakaline magmas which partially crystallized on the wall of a magma chamber and were separated from the residual liquid by diffusive and convective processes. Alternatively, the microgranular inclusions may represent blobs of mafic magmas which, being intruded into a cooler liquid, partially crystallized giving a crystal mush of minerals and interstitial evolved liquid. Concentration of solid phases could have resulted from separation of the interstitial liquid from the crystal aggregates. However, chemical exchanges between the partially crystallized mafic blobs and the host granitoid magmas also played an important role in determining the geochemical composition o f the studied inclusions.