Poster Number 42
See more from this Division: General Discipline Sessions
See more from this Session: Mineralogy/Crystallography; Petrology, Experimental; Igneous; Metamorphic (Posters)
Oxygen and hydrogen isotope analyses have proven to be effective indicators of the sources of granitic magmas, particularly in establishing relative contributions of mantle versus crustal sources. In this study, oxygen isotope ratios of whole rock, quartz, potassium feldspar, and both oxygen and hydrogen isotope ratios of biotite have been measured for the major plutons of the Mid-Proterozoic Llano uplift, central Texas in order to determine the sources of the A-type granites. These syn- and post-kinematic plutons were emplaced 1.19 – 1.07 Ga ago along the southern margin of the Laurentian continent during the Grenvillian orogeny.
Values for quartz separates are +8.4< d18O>+11.6, mean +9.4 (n=37); potassium feldspar: +7.0< d18O>+10.2, mean +8.1 (n=37); biotite: +2.0< d18O>+5.1, mean +3.6 (n=26) and -77.1<dD<-104.8, mean -93.0. Measured fractionations between quartz and potassium feldspar separates are +0.1< d18Oqtz-fsp<+2.6, mean +1.4, suggesting that subsolidus open system exchange was limited for these rocks.
Mantle-derived granites or those
containing a combination of both mantle and crustal derived melts commonly
display d18O values between
+6 and +7.8 (