A. Dana Johnston

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

AbstractPeraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H2O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction:

Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered}

Fluid absent melting of a layered crustal protolith: implications for the generation of anatectic granites

have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H2O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al2O3, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute).The liquids produced in our experiments are silicarich (68–73 wt% SiO2), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO* +TiO2 less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids.We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.

Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H2O at 15 kbar

Vapor-absent melting experiments on a biotite- and amphibole-bearing, Archean tonalitic gneiss (AGC150) at 10 kbar and 875 to 1050 °C show that amphibole breaks down from 900 to 950 °C, producing garnet, orthopyroxene, and granitic melt. Biotite-dehydration melting produces <10 wt% melt up to 950 °C via incongruent melting reactions that produce garnet, orthopyroxene, and titanomagnetite. Widespread biotite-dehydration melting occurs between 950 and 975 °C and produces orthopyroxene, magnetite, titanomagnetite, and ∼20 wt% fluorine-rich melt (up to 0.31 wt% F). Minor F-rich (2.7 wt%) biotite is present even at 1000 °C. Our experiments show that, under vapor-absent conditions, intrusion of hot, mantle-derived magmas into the lower crust is necessary to initiate widespread biotite-dehydration melting in rocks with compositions like AGC150. We propose that the high thermal stability of biotite in AGC150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat F-enriched biotite. We show that dehydration melting of such F-enriched biotite produces F-rich granitic liquids, with compositions within the range of A-type granite, and leaves behind a granulitic residue consisting of orthopyroxene, plagioclase, quartz, titanomagnetite, and magnetite.

The system tonalite-peridotite-H2O at 30 kbar, with applications to hybridization in subduction zone magmatism

The generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world, including the late Mesozoic Sevier belt of western North America. We present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere (i.e. advection of mass and energy are excluded). We distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes, between anatexis and regional deformation and between anatexis and metamorphism. A fundamental conclusion is that, if protoliths rich in hydrous minerals are present, regional anatexis is the end-product of classical Barrovian metamorphism in thickened continental crust, even in the absence of a free water-rich fluid phase. Barrovian metamorphic facies series are predicted with thickening ratios (maximum crustal thickness attained/initial crustal thickness) as low as 1.3, but mobile granitoid magmas are not formed if this ratio is less than approximately 1.5. Above these lower bounds, Barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished. Both processes are sensitive to a diminished heat supply; lowering either Moho heat flow or crustal radioactive heat production results in blueschist-eclogite metamorphism and inhibits the formation of mobile granitoid magmas. We model anatexis under fluid-absent conditions and show that, with such a constraint, migmatization is always a syn-kinematic process (relative to the crustal thickening event), whereas generation of mobile granitoid magmas is in most cases post-kinematic (relative to crustal thickening) but can be syn-kinematic if thickening takes more than approximately 50 Myr. The typical time intervals for melting are consistent with geological observations; mobile granitoid magmas are predicted by most of our models within approximately 10 Myr of the end of the crustal thickening event. This “incubation period” results primarily from the temperature increase required for the dehydration-melting reactions capable of producing large melt fractions to occur. The energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets, as shown by the fact that once the necessary P-T conditions are attained, melting reactions are completed within time intervals on the order of 1 Myr, i.e. 1–2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening.

Experimentally determined rare-earth element and Y partitioning behavior between clinopyroxene and basaltic liquids at pressures up to 20 kbar

We report results of anhydrous 1 atm and piston-cylinder experiments on ID16, an Aleutian high-magnesia basalt (HMB), designed to investigate potential petrogenetic links between arc high-alumina basalts (HABs) and less common HMBs. ID16 is multiply saturated with a plagioclase/spinel iherzolite mineral assemblage (olivine, plagioclase, clinopyroxene, orthopyroxene, spinel) immediately beneath the 12 kbar liquidus. Derivative liquids produced at high temperatures in the 10–20 kbar melting interval of ID16 have compositions resembling those published of many moderate-CaO HABs, although lower-temperature liquids are poorer in CaO and richer in alkalies than are typical HABs. Isomolar pseudoternary projections and numerical mass-balance modeling suggest that derivative melts of ID16 enter into a complex reaction relationship with olivine at 10 kbar and 1,200° C–1,150° C. We sought to test such a mechanism to explain the lack of liquidus olivine in anhydrous experiments on mafic high-alumina basalts such as SSS. 1.4 (Johnston 1986). These derivative liquids, however, do not resemble typical arc high-alumina basalts, suggesting that olivine-liquid reaction does not account for Johnstons (1986) observations. Instead, we suggest that olivine can be brought onto the liquidus of such compositions only through the involvement of H2O, which will affect the influence of bulk CaO, MgO, and Al2O3 contents on the identity of HAB liquidus phases (olivine or plagioclase) at pressures less than ∼12 kbar.

Near-liquidus phase relations of an anhydrous high-magnesia basalt from the Aleutian Islands: Implications for arc magma genesis and ascent

We report the result of H2O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded ∼40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only ∼5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded ∼50 vol% and ∼7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was ∼10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by ∼25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of “minimum-temperature” granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.

Off-center hot spots; double thermocouple determination of the thermal gradient in a 1.27 cm (1/2 in.) CaF 2 piston-cylinder furnace assembly

We report the T-X(H2O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths. Under H2O-saturated conditions, a melting interval of ∼135° C separates the solidus at ∼610° C from the liquidus at 745° C. H2O-saturation at 15 kbar occurs at approximately 15.5 wt % H2O. The H2O-undersaturated liquidus extends along a curved path from ∼745° C at 15.5 wt % H2O to ∼1100° C at 2% H2O. Lower H2O contents were not investigated. At low H2O contents (<6%) sodic plagioclase (Pl, An32) is the liquidus phase followed at lower but still near-liquidus temperatures by quartz (Qz) and then garnet (Ga). At 6% H2O, Ga replaces Pl on the liquidus and is joined at slightly lower temperatures by Pl and hornblende (Hb). The field for liquidus Ga extends to only ∼7.5% H2O where it is replaced by Hb which is the liquidus phase up to 13% H2O. At all higher H2O contents, epidote (Ep) is the first phase to crystallize, followed by biotite (Bi) at slightly lower temperatures. Following the standard inverse approach, the near-liquidus phase assemblages are interpreted as potential residues from which trondhjemitic melts could be extracted. At high melt H2O contents (>7%), mafic residues consisting of some combination of Hb, Ga, Ep, and Bi are possible and could correspond to amphibolitic source rocks. At lower melt H2O contents (< 5%), possible residues consist of Na-Pl+Qz±Ga and could correspond to an earlier generation of tonalitic-trondhjemitic rocks. However, such residues would not impart the highly fractionated REE patterns characteristic of Archean trondhjemites. If a first generation of tonalitic-trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth, it would crystallize to an assemblage consisting almost entirely of Na-Pl+Qz with highly fractionated REE patterns. These rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated REE patterns because neigher Pl nor Qz is capable of significantly fractionating HREE from LREE.