Zoning in hornblende as a key to petrologic evolution of sub-volcanic granitic plutons

Abstract:

The late stage of the Late Jurassic English Peak pluton is inwardly-zoned from tonalite to granite with magnesiohornblende and biotite as principal mafic phases. The late stage is subdivided into three inward-younging units: border, Yellow Jacket Ridge (YJR), and interior Chimney Rock (CR) units. Contacts are gradational and bulk-rock data support either batch-wise emplacement or inward differentiation.

Hornblende (hbl) has olive-brown cores surrounded and embayed by green rims. Al2O3 and TiO2 decrease (9.0–4.5 and 2.5–0.5 wt%, respectively) and MgO increases (10–14 wt%) from cores to rims, with a distinct gap at ~1.5% TiO2, corresponding to the core-rim boundary. Al-in-hbl barometry suggests core crystallization at ~4.2 kbar and rim growth at ~2.4 kbar, probably in a sub-volcanic reservoir.

Most trace element abundances decrease from cores to rims, with Sr, Ba, Hf, and Cr showing linear variation with Zr, with a distinct gap at ~1.5% TiO2 (50-55 ppm Zr). Nb is ~ constant within olive-brown cores and decreases in green rims. In contrast, most 3+ cations (Sc, V, Y, REE) show initial rim-ward decrease, but reach constant values at ~80 ppm Zr, within olive-brown cores. At the core-rim transition in YJR and CR hbl, these elements then increase in abundance. Zoning patterns of hbl from each unit are distinct in terms of Sc, Y, Mn, and REE; each late-stage magma evolved separately, even if they shared a common source.

Trace element modeling using a narrow range of bulk D and hbl/melt d values indicate fractional crystallization explains near-linear core–rim decreases in most trace elements. Bulk partition coefficients were constant or increased sympathetically. Slope changes at ~80 ppm Zr can be modeled as increases in hbl/melt partition coefficients as T decreased. This change occurs within the core zones, presumably prior to rise to the level of emplacement. The abrupt increases in 3+ cation abundances in green rims can be modeled as the result of core resorption accompanied by overgrowth of green rims.

The three late-stage units represent distinct magma pulses into a shallow crustal reservoir, potentially from a common, mid-crustal reservoir. The pluton is likely to have fed a composite volcano with multiple dacitic/rhyodacitic domes fed from the shallow reservoir.