Allen J. Schaen

Complementary crystal accumulation and rhyolite melt segregation in a late Miocene Andean pluton

High-silica granites are hypothesized to form via fractionation in the shallow crust, yet the predicted residues are rarely identified and can be difficult to distinguish within plutons whose rocks otherwise plot along liquid lines of descent. Bulk-rock compositional mass balance in the late Miocene Risco Bayo–Huemul plutonic complex (Chile) suggests that lithological differences within the Huemul pluton reflect residual crystal concentration in response to melt extraction. A compositional gap from 70 to 75 wt% SiO2 and strong depletion in Ba and Eu suggest that Huemul alkali feldspar (Afs) granites are frozen remnants of highly evolved rhyolitic melt extracted from a mush. Quartz monzonites enriched in Zr and Ba with Eu/ Eu* near unity are interpreted to represent the complementary residual silicic cumulates of this fractionation process. Compositional variations in Afs granite zircon (Eu/Eu*, Dy/ Yb) further support extraction of this melt from a zircon-saturated mush. U-Pb zircon dates indicate that Huemul rocks evolved ~800 k.y. after initial crystallization of more mafic Risco Bayo rocks, likely precluding their evolution via fractionation from mafic forerunners. This pluton records a means to produce rhyolite in the upper crust, which has propelled large silicic eruptions during the Quaternary within the Andean subduction zone.

More stable yet bimodal geodynamo during the Cretaceous superchron

We report palaeomagnetic and Ar-40/Ar-39 dating results from two sequences of basaltic lava flows deposited at the same locality in western China, yet separated in time by similar to 50 Myr: one set lies within the Cretaceous normal superchron at 112-115 Ma and a second at 59-70 Ma spanning the Cretaceous-Palaeogene boundary. We find that magnetic field directions during the superchron exhibit bimodal populations: one with inclinations representative of a dipolar field and another with shallow inclinations that could reflect a more complex, multipolar field. However, the time-dependent variability in field directions was 50% lower during the superchron than after, which implies greater field stability during the superchron. Our results suggest that episodes of less dipolar field behavior occurred within the Cretaceous superchron and raise the question whether a second, more multipolar, field state is more persistent than previously thought.

Textural and Mineralogical Record of Low Pressure Melt Extraction and Silicic Cumulate Formation in the late Miocene Risco Bayo-Huemul Plutonic Complex, Southern Andes

One model for rhyolite generation in the upper crust is via extraction of interstitial melt from crystal-rich magma reservoirs. Although silicic magma reservoirs may grow incrementally over c. 10–10 yr timescales, they can be remobilized prior to eruption much more rapidly (c. 10–10 yr). This process implies the formation of cumulate residues with a composition complementary to the extracted melt, but the predicted cumulates have so far eluded widespread identification. The 7 2– 6 2 Ma Risco Bayo–Huemul plutonic complex comprises 150 km that is subdivided into at least seven gabbroic to granitic domains emplaced at <7 km depth. Distinct Ti, Zr, Nb, and rare earth element variations in amphibole document pulsed emplacement of the gabbroic to granodioritic Risco Bayo pluton 800 kyr prior to the adjacent quartz monzonite to high-silica granite of the Huemul pluton. The quartz monzonite is inferred to be a silicic cumulate on the basis of whole-rock mass balance and enrichments in Ba and Zr concentrations. Here, we combine fine-scale textural analysis using energy-dispersive X-ray spectrometry (EDS) phase mapping with in situ mineral compositions to explore the silicic cumulate hypothesis. Quartz monzonite textures are porphyritic and comprise 58–64 modal % of partially interlocking, 2–5 mm long euhedral plagioclase, together with euhedral biotite, orthoclase, and amphibole. The finer-grained interstitial matrix is composed of anhedral orthoclase, plagioclase, and quartz. Calculations using the compositions of the interstitial phases suggest that the matrix represents a highly evolved melt similar in major and trace element chemistry to coeval, high-silica granite inferred to be extracted and frozen rhyolite. The high-silica granites are equigranular and contain dense concentrations of miarolitic cavities implying, together with Al-in-Hbl barometry for the quartz monzonite, emplacement and volatile saturation within the upper crust at the granite minimum. Plagioclase, orthoclase, and biotite in high-silica granite are depleted in Ba, Sr, and Eu, similar to their bulk-rock compositions, and support an origin as highly fractionated products of melt extraction. Miarolitic cavities within the high-silica granite overlying the granite domain suggest that volatiles played an important role in the upward percolation of melt. Our observations indicating transport of volatiles and melt through an underlying crystal mush, including accumulation of vapor bubbles at the roof of the magma VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1991 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 10, 1991–2016 doi: 10.1093/petrology/egy087 Advance Access Publication Date: 7 September 2018