Fertility of metapelites and metagraywackes during leucogranite generation: an example from the Black Hills, U.S.A.

Abstract

In order to elucidate how mineralogy and composition of crustal sources influences production of leucogranite magmas, we modelled the potential fertility of a sequence of metapelites and metagraywackes from the Black Hills, South Dakota, U.S.A., using a least-squares mixing approach. Rocks analogous to the Black Hills schists were the sources of the Harney Peak leucogranite. Both muscovite and biotite fluid-absent melting reactions (MM and BM, respectively) were investigated. Using the Harney Peak Granite composition as the melt analogue and mineral compositions from the schists for mixing calculations, it is shown that MM of metapelites would lead to highly variable residue mineralogy in the investigated samples. The average residue includes 36 wt.% biotite, 32 wt.% quartz, 12 wt.% plagioclase, 8 wt.% K-feldspar, 9 wt.% sillimanite and 2 wt.% garnet. Melt production ranges from 5% to 23% with an average of 14%. It is limited by the amount of H2O that must be in the melt at the conditions of melting, relative to the amount that is in muscovite in the source rocks. Plagioclase-rich metagraywackes contain little to no muscovite, thus MM cannot occur in them.Although BM is continuous over a wide temperature range, for the purposes of modelling melting at 975°C and 10kbar was chosen. The temperature is near the terminal stability of biotite, thus the calculations give near-maximum melt production. At this temperature, the mineralogy of the model residues from both metapelites and metagraywackes is dominated by garnet. The potential melt production in the metapelites ranges from 0% to 58% with an average of 32%. It is limited by the availability of plagioclase in the source rocks. Potential melt production in the metagraywackes ranges from 9% to 37% with an average of 23%. At the chosen conditions of melting, melt production is limited by the available K in biotite, although at lower temperatures, the available H2O limits melt production. The total potential melt production (MM + BM) in the metapelites is higher because they have on average a low normative An/Ab ratio (0·14) that approaches the ratio in the leucogranites (0·04). The paragonite component in muscovite significantly contributes to the low ratio in the metapelites. The higher ration (0·27) in the metagraywackes is denned by the feldspar composition.Using the calculated melt fractions and residue mineralogies, we modelled the concentrations of Rb, Sr and Ba in the melts, as these elements are important indicators of melt-generating processes. The results indicate that both Sr and Ba are likely to be heterogeneous in extracted melt batches and will be depleted in partial melts relative to their pelitic sources, irrespective of whether the melting is fluid-absent or fluid-present.

The initial garnet‐in reaction involving siderite–rhodochrosite, garnet re‐equilibration and P–T–t paths of graphitic schists in the Black Hills orogen, South Dakota, USA

“…The bulk composition for which the pseudosections were constructed is an average of 33 metapelites representing all metamorphic grades and sampled formations (Table ). The metapelites have a very narrow range of Fe/Mg ratios (1.64 ± 0.26; Nabelek & Bartlett, ), but MnO ranges from 0.04 to 0.76, roughly correlating with the amount of garnet. The host rock for garnet G‐227‐1 is an outlier with 1.6 wt% MnO (Table ).…”

Insights into the metamorphic history and origin of flake graphite mineralization at the Graphite Creek graphite deposit, Seward Peninsula, Alaska, USA

“…The retrograde event likely caused localized recrystallization of graphite via influx of fluids. Bt biotite, Gr graphite, Grt garnet, Sil sillimanite, Tur tourmaline, MC metamorphic complex composition and water content of the metapelite protolith (Nabelek and Bartlett 2000). However, melt removal may not fully account for graphite enrichment.…”

Petrologic and thermal constraints on the origin of leucogranites in collisional orogens

“…Partial melting experiments on metapelites (Patiñ o-Douce & Harris 1998), and structural relationships between migmatites, melt ascent paths and plutons (Le Fort et al 1987;Brown & Solar 1998b) support this link. The often significant paragonite component in muscovite adds to melt production (Nabelek & Bartlett 2000). The often significant paragonite component in muscovite adds to melt production (Nabelek & Bartlett 2000).…”

“…Metapelites are more fertile melt producers than metagraywackes because metapelites contain higher proportions of muscovite. The often significant paragonite component in muscovite adds to melt production (Nabelek & Bartlett 2000).…”

“…Feldspar removal would increase Sr concentrations in the melt above those which are observed. However, it is cautioned, that the relative ratios are also dependent on the amount of biotite in the residue, whose proportion during MDM can be sufficiently high to cause lower Rb:Sr and higher Sr:Ba ratios in partial melts than their source rocks, even when plagioclase remains in the residue (Nabelek & Bartlett 2000). (2) Patiñ o-Douce & have shown experimentally that fluid-present melting of Himalayan metapelites produces melts which are trondhjemitic in composition, whereas fluid-absent melting produces melts whose compositions match those of the Himalayan leucogranites.…”