Paper No. 1
Presentation Time: 8:00 AM-6:00 PM
SPHENE AS A PRODUCT OF CALC-SILICATE ASSIMILATION, HORTAVæR INTRUSIVE COMPLEX, NORWAY
BARNES, Calvin G., Geosciences, Texas Tech, Lubbock, TX 79409-1053, LI, Yujia, Texas A&M University, Department of Geology and Geophysics, College Station, TX 77843, PRESTVIK, Tore, University of Trondheim, Dept. of Geology and Mineral Resources Engineering, Trondheim, NO-7491, Norway and BARNES, Melanie A., Department of Geosciences, Texas Tech University, Lubbock, TX 79409, cal.barnes@ttu.edu
The Hortavær intrusive complex in the Caledonides of north-central Norway is a layered suite of gabbroic through syenitic rocks with a capping unit of alkali feldspar granite. The gabbroic through syenitic magmas evolved to alkaline compositions via pervasive, locally intense assimilation of calc-silicate rocks. Assimilation was in part related to partial melting of the calc-silicates and subsequent mixing of Ca-rich melts; residual magmatic skarns are syenitic to monzonitic in composition, and also contain calcite + sphene ± garnet ± wollastonite. Assimilation also occurred via reaction assimilation to form Al-rich diopside–hedenbergite, Ca-rich amphibole, and sphene, and locally nepheline, garnet, scapolite, and vesuvianite. Sphene is ubiquitous from gabbro to syenite and in the magmatic skarns; it varies from interstitial in the gabbros to euhedral, 5-mm-long crystals in more evolved rocks. With a few exceptions, these rocks lack zircon and contain little or no Fe-Ti oxides.
LA-ICPMS analysis of sphene shows a wide range of trace element abundances and REE patterns. The REE patterns of all samples have negative slopes and most show negative Eu anomalies. Sphene from syenitic rocks (intrusive syenite and magmatic skarns) has the highest Gd/Lu ratios and the lowest Lu abundances. In at least some samples, variation in the slopes of sphene REE patterns can be ascribed to garnet fractionation; in others to sphene fractionation. Sphene from the magmatic skarns has the highest Sr and lowest Y abundances of all samples, which suggests inheritance of these element abundances from calc-silicate protoliths. With the exception of one sample, Zr contents in sphene are 1400–4600 ppm and may vary by 2000 ppm in crystals from a single sample, making sphene the reservoir for Zr in the complex. On a complex-wide basis, sphene fractionation is evident from the decrease in bulk-rock Zr contents from monzodioritic through syenitic samples. This variation suggests relatively high-T sphene stability. We suggest that high-T stability is consistent with the high activity of Ca due to assimilation of calc-silicates. In addition, Zr-in-sphene temperature values of 800–850°C support this idea but are suspect owing to a lack of buffer assemblage.
