Evidence for magma fracturing, solidus depression, coarse magmatic epidote, devitrified and nanogranite melt pockets, and possible magmatic chlorite in the mid-crustal Mount Stuart Batholith, USA

What resemble epidote-mineralized joints in the mid-crustal Late Cretaceous Mount Stuart Batholith (Washington, USA) may instead be evidence for mode I/joint-like fractures in the magma during crystallization. Zones ranging in thickness from a few mm to several cm are occupied by epidote, chlorite, and other minerals. Adjacent to these zones in the hornblende quartz diorite, textures suggest epidote grew into the crystal mush, and was partly replaced and overgrown by magmatic plagioclase. The plagioclase appears to have nucleated on the epidotes, and commonly contain fragments in optical continuity. The plagioclase is nearly pure albite (~An99), and Sr and Y commonly exceed Ca. These relationships do not resemble low T post-magmatic alteration of the plagioclase.

In the same areas, rounded to amoeboid patches, up to several mm across, are composed of small (25-100 micron) and uniform ‘pills’ of hemispherical or radiating chlorite aggregates (xMg~0.7) occur, in sharp contact with adjacent minerals. Some are completely enclosed inside other minerals including quartz and plagioclase, whereas others are interstitial to large igneous minerals. Some contain grains of apatite, titanite, or fragments of epidote. These are tentatively interpreted to have originated as melt patches, from a melt greatly depleted in Ca, Si, Na, and K from the crystallization of plagioclase, quartz, and other minerals, and likely in the process of crystallizing apatite and titanite. A few ultra-fine-grained patches suggest a possible glass precursor. Assuming simple hydration, it would have been a very mafic residual melt.

Complex intergrowths of minerals are common, including a partial replacement of epidote by hornblende. Fluid inclusions are large and abundant in multiple minerals.

There is abundant evidence of halogens, including fluorapatite, and Cl, F, Br, and even I detectable in several minerals. REEs are detectable even by EDS in several minerals. K-feldspar contains up to 1.5 wt% Ba.

Large, healthy chlorite crystals comparable in size to the magmatic minerals occur in sharp contact with other minerals, and these do not appear to be replacing anything, nor does chlorite like this occur elsewhere in the batholith.

A 0.3 mm zircon contains a small ovoid patch containing quartz, K feldspar, and a more calcic plagioclase. This is interpreted as a melt patch of an earlier composition, crystallized into a nanogranite.

Tentatively, these relationships suggest fracturing of the crystal mush during crystallization led to a water- and halogen-rich ‘dike’ that interacted with the adjacent melt, dropping the solidus. Epidote crystals, some multi-cm in length, penetrated the melt, but was subsequently partly resorbed during crystallization of hornblende, plagioclase, and quartz, and even possible magmatic chlorite, at a depressed solidus temperature, and remaining melt quenched into the pockets. Fluid inclusion work and geothermometry is in progress.