Calvin F. Miller

Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites

The generation of trondhjemites and tonalites on a massive scale during the Archean (3.8-2.5 Ga ago) marked the transition from a simatic to a sialic crust, and represents the magmatic contribution to cratonization. Petrogenetic models for the origin of these rocks based on their highly fractionated, HREE-depleted rare earth patterns suggest a mafic crustal source, either through a process of partial melting of amphibolite, garnet-amphibolite, or eclogite, in which hornblende and/or garnet are essential residual phases, or by hornblende-controlled fractionation of hydrous basaltic magma. A series of vapor-absent (i.e., Pfluid< Ptotal) melting experiments on four natural basaltic compositions were conducted at 8, 16, 22 and 32 kbar in order to assess the validity of models for the origin of Archean granitoids which assume a mafic crustal source. Melt compositions produced by 10–40% melting are tonalitic-trondhjemitic at all pressures investigated; residual assemblages are amphibole+plag±opx±FeTi at 8 kbar, garnet+cpx±amphibole±plag±opx±FeTi oxide at 16 kbar, and garnet+cpx±rutile at 22 and 32 kbar. REE patterns for most of the trondhjemitic-tonalitic partial melts, calculated on the basis of estimated modal proportions of melt and residual phases, are highly fractionated (LaYb is 30–50), heavy rare earth-depleted (YbN is 1–10) when garnet is present to some extent in the residue; these REE patterns are similar to those of trondhjemitic and tonalitic gneisses from several Archean “grey gneiss” and granite-greenstone terrains. A consideration of estimated Archean geotherms with respect to the experimental P-T conditions indicates that a temporally diminishing Archean geotherm might have progressively swept through a P-T regime in which trondhjemitic-tonalitic melts could have been generated initially from a water-saturated (Pf=Pt) to undersaturated (Pf<Pt) amphibolite source by partial melting at 5–8 kbar. Subsequent relaxation of the geotherm through the mid- to late-Archean would have produced similar melts by vapor-absent melting of garnet-amphibolite at 16 kbar and eclogite at 22–32 kbar. However, the degree of melting required to produce melts of trondhjemitic-tonalitic composition increases with pressure, 10–15% melting being appropriate at 8 kbar in a amphibole-dominated residue, but 25–35% melting being required at 22–32 kbar, where garnet dominates the residue. Supposing the tendency for melt segregation and/or magma mobilization mechanisms to be more effective at higher degrees of melting, an origin by partial melting of eclogite seems to be the most likely source for massive plutonic trondhjemite-tonalite contributions to the juvenile continents. Such a source is consistent with the generation of trondhjemite-tonalite protocontinental cores in any number of plausible Archean tectono-thermal scenarios, though not necessarily in a conventional subduction-zone setting.

Hot and cold granites? Implications of zircon saturation temperatures and preservation of inheritance

Zircon saturation temperatures ( T Zr) calculated from bulk-rock compositions provide minimum estimates of temperature if the magma was undersaturated, but maxima if it was saturated. For plutons with abundant inherited zircon, T Zr provides a useful estimate of initial magma temperature at the source, an important parameter that is otherwise inaccessible. Among 54 investigated plutons, there is a clear distinction between T Zr for inheritance-rich (mean 766 °C) and inheritance-poor (mean 837 °C) granitoids. The latter were probably undersaturated in zircon at the source, and hence the calculated T Zr is likely to be an underestimate of their initial temperature. These data suggest fundamentally different mechanisms of magma generation, transport, and emplacement. “Hot” felsic magmas with minimal inheritance probably require advective heat input into the crust, are crystal poor, and readily erupt, whereas “cold,” inheritance-rich magmas require fluid influx, are richer in crystals, and are unlikely to erupt.

Zircon zonation patterns as revealed by cathodoluminescence and backscattered electron images: Implications for interpretation of complex crustal histories

Abstract Zircon exhibits an extraordinary memory. Its stability, durability, low solubility and low elemental diffusivities combine to preserve in it a record of most of the important events that have affected it, its host rocks, and the crust of which it is a part. Zonation in zircon grains delineates the boundaries of discrete geochemical packages formed at different times, each effectively a closed system. The elemental and isotopic compositions of these packages reflect the timing and conditions of growth events, and the morphology of the zonation indicates qualitatively the nature of both growth and intervening degradation events. Cathodoluminescence (CL) and backscattered electron (BSE) imaging reveals detailed zonation patterns that are commonly invisible or barely visible with conventional transmitted and reflected light microscopy. Characteristic patterns are visible in almost all zircons that serve to distinguish igneous from metamorphic growth, to distinguish truncation surfaces of different types (e.g., sedimentary fracturing vs. resorption), and possibly to identify ancient metamictization. Zircons from many rocks record multistage histories that reflect two or more events; those from rocks such as peraluminous granites and high-grade paragneisses are especially likely to reveal long and complex histories. Studies of zonation patterns in zircons provide a clear, though qualitative, history of a rock and its heritage. Furthermore, they provide the basis for a quantification of that history. Elemental and isotopic compositions can reveal the environment in which a zone grew. U-Pb analysis of a zone provides an age for its growth. shrimp analyses that are not guided by detailed knowledge of zonation can straddle two (or more) zones and a discordant U-Pb result from such an analysis may falsely suggest Pb loss, and important growth zones may be missed entirely. Thus, the combined use of CL, BSE, electron microprobe and ion probe methods can elucidate complex crustal histories.

Are Strongly Peraluminous Magmas Derived from Pelitic Sedimentary Sources

Igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous (Ps). Such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks. The term S-type has been applied to many igneous rocks based largely or even solely upon their Ps compositions, neglecting or placing less emphasis on other criteria for sedimentary parentage in Chappell and Whites I- and S-type classification system. The S-type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature (strongly weathered-pelitic, shaly) source. Application of Chappells and Whites criteria, together with others based upon isotopic and phase equilibria considerations, leads to the following conclusions about Ps igneous rocks worldwide: (1) very few Ps rocks are derived entirely from pelitic sources; (2) a majority are derived largely from intermediate to felsic crustal sources, including both immature sedimentary rocks (e.g., metagraywackes) and metaigneous rocks; and (3) some are the products of partial melting of metaluminous mafic (crustal or subcrustal) sources-Ps compositions of such magmas may be a primary feature or a result of fractional crystallization.

Constraints on timing of peak and retrograde metamorphism in the Dabie Shan Ultrahigh-Pressure Metamorphic Belt, east-central China, using U–Th–Pb dating of zircon and monazite

Abstract The Dabie Shan Ultrahigh-Pressure Metamorphic (UHPM) Belt occupies the suture between the Yangtze and Sino-Korean blocks in east-central China. The timing of UHPM in the Dabie belt is controversial, and most recent data come from dating of zircons. Monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones, and monazite growth has been documented to occur during prograde, peak, and retrograde metamorphism. Zircons and monazites from UHP mafic rocks from Maowu and a UHP jadeite quartzite from Shuanghe were imaged in thin sections and in mineral separates and dated using a high-resolution ion microprobe. Maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite. Maowu eclogites and garnet pyroxenites contain zircons with mean 206 Pb/ 238 U age of ∼230±4 Ma, and monazites from a clinopyroxenite have 208 Pb/ 232 Th ages of ∼209±4 Ma. Multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages (i.e., they do not correspond to the cessation of diffusional Pb loss as rocks cooled below the closure temperature during exhumation). Shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921±22 Ma and lower intercept age of 236±32 Ma, interpreted to represent the ages of the source rocks and of peak metamorphism, respectively. Zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1.5 GPa. Their pooled age of 238±3 Ma agrees well with the lower intercept defined by cores, suggesting that Pb loss from cores and growth of new rims occurred during UHPM at ∼235–240 Ma. Monazite records multiple events during retrograde metamorphism. Maowu clinopyroxenite and Shuanghe jadeite quartzite experienced monazite growth at ∼209 Ma, interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration. Only the jadeite quartzite records growth at 223±1 Ma. Estimated exhumation rates for Maowu are 7–8 km/Ma.

Depletion of light rare-earth elements in felsic magmas

Contrary to simple generalizations about their behavior, light rare-earth elements (LREE) do not act as incompatible elements in very felsic magmas. In fact, LREE concentrations typically decrease, often drastically, during differentiation of such magmas. The simplest explanation for this depletion involves the separation of minute, easily overlooked quantities of an extremely LREE-rich accessory mineral, either monazite or allanite. Our data indicate that felsic liquids with < 50 ppm LREE may be saturated in either of these accessories and that the concentration required for saturation decreases in increasingly felsic liquids. This accounts for incompatible behavior of LREE even at high concentrations in mafic magmas in contrast to compatible behavior at low concentrations in felsic magmas. Partitioning of LREE into solid rather than liquid has important implications for trace-element and Nd-isotope modeling of crustal anatexis, as well as for magma differentiation.

Tracking magmatic processes through Zr/Hf ratios in rocks and Hf and Ti zoning in zircons: An example from the Spirit Mountain batholith, Nevada

Abstract Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ~35-40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15-30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (TTiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ~30-40 for cumulates and 18-30 for high-SiO2granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and TTiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

Geochemistry of the Sweetwater Wash Pluton, California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Abstract Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients. Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks. Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.

Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

Abstract Studies of the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance. Unmetamorphosed Latham Shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition. Monazite is absent in the shale and low-grade schists, but it is abundant in schists at staurolite and higher grades. Lower-grade schists instead include minute Th- and Ce-oxides and unidentified Ce-poor LREE-phosphates that apparently are lower-temperature precursors to monazite. Thus monazite originates when the pelite passes through lower-amphibolite-facies conditions. Monazites from three Upper Cretaceous granites yield ages that are strongly discordant. Upper intercepts of 1.6–1.7 Ga are similar to those defined by U-Pb data for coexisting zircons and coincide with a period of copious magmatism in the Mojave crust. As the host Upper Cretaceous granitic magmas were all above 700°C, effective closure of the restitic monazites to Pb loss must be well in excess of this temperature. U-Pb compositions of monazite from Proterozoic granitoids and schist also indicate high Pb retentivity. Taken together, these studies support the suggestion that monazite can be an effective prograde thermochronometer. At least in pelites, it is not usually retained as a detrital mineral, but rather forms during moderate-temperature metamorphism. Its U-Pb system should not be reset by subsequent higher-grade metamorphism.

Low temperature replacement of monazite in the Ireteba granite, Southern Nevada: geochronological implications

The Ireteba pluton is a relatively homogeneous, ∼64 Ma (zircon ion probe age) two-mica granite that was intruded by two 16 Ma Miocene plutons at depths ranging from 5 to 13 km. Deeper levels of the Ireteba and Miocene plutons were ductilely deformed at 15–16 Ma. At shallow levels remote from the Miocene plutons, the Ireteba granite appears to have experienced little Miocene heating and deformation. Monazites from different portions of the pluton reflect the different histories experienced by the host rock. Irregularly shaped (patchy) zones with high huttonite component (ThSiO4) are widespread in monazite at deep levels adjacent to Miocene plutons but less common in shallow-level rock; monazite grains with extensive replacement generally have irregular, embayed surfaces. In undeformed rocks distant from the Miocene plutons, monazites are less modified and more nearly euhedral, though fine networks of replacement veins are common and irregular rims are evident in some grains. Secondary monazite from these samples is poorer in huttonite. Ion probe Th–Pb dating yields 60–65 Ma ages for magmatic and some replacement zones in monazite from the shallow samples, and veins yield apparent ages as young as mid-Tertiary. Monazites from deep samples yield a few 55–65 Ma ages for remnant magmatic zones and abundant Miocene ages for replacement zones (∼14–18 Ma). These data demonstrate extensive Miocene replacement of magmatic monazite, especially at deep levels near Miocene plutons, and they suggest an early replacement episode as well. Both events were probably related to influxes of fluid; the first may have been associated with initial solidification of the Ireteba pluton and the second with the Miocene plutons and/or extensional deformation. Ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at T as low as 400°C or less.