EVALUATING HIGH SR/Y GEOCHEMICAL SIGNATURES IN LOWER-ARC CRUST USING PYROXENE MAJOR AND TRACE ELEMENT DATA

High-Sr/Y plutonic rocks form major components of Phanerozoic convergent margins, yet their origin remains debated. Sometimes referred to as ‘adakite’ or tonalite-trondhjemite-granodiorite (TTG), high-Sr/Y plutonic rocks are commonly linked to partial melting of subducted oceanic crust or lower continental crust; however, there is no consensus on the petrogenesis of these rocks or the settings and events that triggered their generation. Here, we investigate a suite of high-Sr/Y plutonic rocks from the lower crust of the Median Batholith, South Island, New Zealand. We present whole-rock data and >220 new electron microprobe and laser ablation inductively coupled mass analyses for magmatic clinopyroxene across ~550 km² of the lower crust. Bulk-rock data show that the Malaspina Pluton is characterized by low SiO₂ (50-60 wt.%), Y (<20 ppm), and heavy rare earth element concentrations (Yb <2.0 ppm), and high Al₂O₃ (>18 wt.%), Na₂O (4.0 wt%), Sr (>1000 ppm), and Sr/Y and La/Yb (>50 and >15, respectively). Magnesium numbers (molar Mg/(Mg+Fe), or Mg#) are low and range from 0.3 to 0.6. Clinopyroxene also have fairly low Mg#s ranging from 0.6 to 0.7. They also have mean Sr concentrations of 60 ppm, and Zr and Y values of 117 ppm and 77 ppm, respectively. They display moderately fractionated LREE/HREE patterns with Nd/Yb values ranging from 7-15. Malaspina clinopyroxenes are more evolved when compared to those found in Aleutian basalts and have lower Sr concentrations compared to those in Aleutian adakites. Calculated melts compositions give surprising low average Sr (215 ppm) and Sr/Y (<10) values, and high Y (32 ppm) and Zr (360 ppm). Low Sr/Y values (~35) are also observed in calculated melts derived from magmatic amphiboles in the Malaspina Pluton. These values from clinopyroxenes and amphiboles are strikingly different from bulk-rock values. We suggest that Malaspina melts were evolved, Zr-rich melts with low-Sr/Y values, and bulk-rock values reflect crystal accumulation.