TRACE ELEMENT ZONING IN HORNBLENDE: TRACKING AND MODELLING CRYSTALLIZATION OF CALC-ALKALINE ARC PLUTONS (Invited Presentation)

Major and trace element compositions and zoning patterns in hornblende (Hbl) from calc-alkaline granitic rocks provide a record of the final tens to hundreds of degrees of crystallization. Analyses of major and trace elements made on the same spot allow compositional variation to be ascribed to petrologic processes and changes in T and P. Previous studies on Hbl from the upper zone of the Wooley Creek batholith (Klamath Mtns., NW CA) showed uniform variation of trace elements with decreasing T (820–720°C; [1]). Because this variation is identical across 160 km² of the pluton, crystallization from a chemically unified magma body was inferred. To test this approach, Hbl from the ca. 91 km² late-stage unit of the adjacent English Peak (EP) pluton was analyzed. The late-stage was mapped as three nested intrusive units with gradational contacts (Schmidt, 1994). In each, Hbl is euhedral to subhedral, with prominent olive-brown core zones which crystallized at ca. 400 MPa from 880–775°C. Cores are embayed and rimmed by green Hbl that grew at ca. 200 MPa from 775–690°C. Some trace elements (Zr, Hf, Sr, V) decrease monotonically from core to rim, suggesting coprecipitation of plagioclase, ilmenite, and zircon. Others (Cr, Ba, Rb) increase slightly in highest-T core zones, then decrease, consistent with onset of biotite crystallization. In contrast, initial outward decreases in Sc, Y, and REE change to near-constant values within olive-brown cores, a change that can be modelled by a decrease in bulk partition coefficients (D). These elements then increase in green rims, with as much as 4X enrichment. The enrichment cannot be due solely to decreased D, and is probably the result of resorption/reprecipitation owing to decreased P, whereby trace elements in core zones were redistributed to the rims. Although Hbls from the three mapped zones are similar, Hbl in the outer zone has higher Ti, Ba, Sc, and REE whereas in the interior zone it has higher Mn. These differences are consistent with episodic magma emplacement from a mid-crustal reservoir. Rim-ward decrease in Zr indicates zircon and Hbl crystallized together, at T as high as 880°C. Because the zircon saturation T is < 720°C for all EP samples, we infer that most or all samples are cumulates.

[1] Putirka, 2016, Am. Min.; [2] Schmidt, 1994, Ph.D. diss. Texas Tech.