Michael Brown

University of Maryland, College Park | UMD, UMCP, University of Maryland College Park · Department of Geology

Using petrography, in situ garnet Lu-Hf geochronology, garnet rare-earth element (REE) analysis, zircon U-Pb geochronology and phase equilibrium modelling, we provide unambiguous evidence for Eoarchean granulite-facies metamorphism in the northern Itsaq Gneiss Complex (IGC), southwest Greenland. In situ garnet Lu-Hf geochronology from two samples o...

To constrain the rate of cooling of lower-crustal rocks from an ultrahot orogen, we determined both the age and equilibration temperature of metamorphic zircon from six widely spaced samples of metasedimentary garnet−sillimanite gneiss from the Eastern Ghats Province in eastern India. For the combined data set of metamorphic zircon, concordant date...

The petrogenesis of contemporary igneous and metamorphic rocks is commonly explained by plate tectonics, but how far back in time does this relationship hold? Here we investigate whether the distinctive petrological features of recent ocean crust, subduction-related magmatism and regional metamorphism can be unambiguously identified in the Archean...

Melt/fluid evolution processes are important in determining the rheological behavior of lithosphere subducted to and exhumed from ultrahigh-pressure (UHP), controlling the flow of lithospheric material, the generation of melts from the upper mantle, and potentially contributing to magmatism and growth of the continental crust. In these circumstance...

The oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in northwestern Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamo...

The past 40 years have been a golden age for eclogite studies, supported by an ever wider range of instrumentation and enhanced computational capabilities, linked with ongoing developments in thermobarometry and geochronology. During this time, we have made robust estimates of pressure–temperature (P–T) conditions; determined ages related to the pr...

We integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strengt...

The thermal state of the mantle impacts lithospheric dynamics and Earth's surface evolution. Geodynamic simulations predict that aggregated continents may act as an effective thermal insulator leading to the warming of the underlying mantle (mantle warming hypothesis). Mantle warming weakens the continental lithosphere, which should lead to more di...

When it comes to convection, what goes up must come down. Or is it, what goes down must come up? The truth is it depends. Although convection must be mass balanced, there is no reason that it must be force balanced: the positive and negative buoyancy forces driving convection up and down, respectively, do not necessarily need to be balanced. The ba...

Earth is the only planet known to have continents, although how they formed and evolved is unclear. Here using the oxygen isotope compositions of dated magmatic zircon, we show that the Pilbara Craton in Western Australia, Earth’s best-preserved Archaean (4.0–2.5 billion years ago (Ga)) continental remnant, was built in three stages. Stage 1 zircon...

When disturbed, dynamic emergent systems, such as tectonics on Earth, may transition from one stable state to another if the perturbation is sufficiently large. Here, we identify such state shifts through examination of statistically-significant change points in the time series of metamorphic pressure–temperature and cooling rate data. Change point...

The rates and mechanisms by which deeply subducted continental crust was exhumed back to the surface are not well understood, but can be better characterized using multimineral petrochronology. Here, we combine zircon, titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with a pressure−temperature (P−T) path from eclogite to provi...

Since the Jurassic, there has been a clear spatiotemporal correlation between different types of metamorphism and active convergent plate margins. However, the extent to which this relationship extends into the past is poorly understood. We compared paleogeographic reconstructions and inferred plate kinematics with the age and thermobaric ratio (te...

Since the Archean, secular change in orogenic style is demonstrated through evolution of metamorphic conditions and geochemical proxies. Linked to orogenic style is the amount of crustal thickening and elevation, whereas orogenic vigor is related to the supercontinent cycle. An array of Proterozoic orogens spanned the assembly of supercontinents Co...

Partial melting is the fundamental process by which juvenile crust was produced from the mantle and subsequently reworked to become the stable, compositionally-differentiated continents on which we live and which host most of the elemental resources required by our modern technological society. Irreversible differentiation of the continental crust...

Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features an...

Robust quantification of pressure (P)–temperature (T) paths for subduction‐related HP/UHP metamorphic rocks is fundamental in recognizing spatial changes in both the depth of detachment from the down‐going plate and the thermal evolution of convergent margin sutures in orogenic belts. Although the Chinese southwestern (SW) Tianshan is a well‐known...

If we accept that a critical condition for plate tectonics is the creation and maintenance of a global network of narrow boundaries separating multiple plates, then to argue for plate tectonics during the Archean requires more than a local record of subduction. A case is made for plate tectonics back to the early Paleoproterozoic, when a cycle of b...

A time-series analysis of thermobaric ratios (temperature/pressure [T/P]) for Paleoarchean to Cenozoic metamorphic rocks identified significant shifts in mean T/P that may be related to secular change in the geodynamics on Earth. Thermobaric ratios showed significant (>95% confidence) change points at 1910, 902, 540, and 515 Ma, recording drops in...

Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UH...

Metamorphic rocks—rocks that undergo transformations in response to changes in pressure (P) and temperature (T)—are a primary source of information for investigating tectonic processes. In their major and accessory mineral assemblages, compositions and microstructures, they provide a temporal record of metamorphism that informs us about our planet'...

The exotic Haiyangsuo complex is structurally part of the Sulu belt but its contact relationship with surrounding Sulu gneisses is unexposed and therefore unknown, making its affinity uncertain. It comprises gneisses with in-source leucosomes that host minor metabasite bodies; both are cut by leucogranite dikes. In this study, we determine the timi...

In the eastern Himalayan syntaxis, the Indus–Tsangpo Suture Zone separates the Himalaya to the south from the southern Lhasa terrane to the north. In this study, we combine petrology, zircon U–Pb geochronology and phase equilibrium modeling to quantify the P–T–t paths of metapelites from the Namche Barwa complex, part of the Himalaya, and the Nying...

Subduction is a component of plate tectonics, which is widely accepted as having operated in a manner similar to the present-day back through the Phanerozoic Eon. However, whether Earth always had plate tectonics or, if not, when and how a globally linked network of narrow plate boundaries emerged are matters of ongoing debate. Earth's mantle may h...

Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics¹. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science1–4. Metamorphic rocks—rocks that have experienced solid-state mineral transformati...

Plate tectonics is among the most important geological processes on Earth, but its emergence and evolution remain unclear. Here we extrapolate models of present-day plate tectonics to the past and propose that since about three billion years ago the rise of continents and the accumulation of sediments at continental edges and in trenches has provid...

The 51st Hallimond Lecture Time's arrow, time's cycle: Granulite metamorphism and geodynamics - Michael Brown, Tim Johnson

On contemporary Earth, subduction recycles mafic oceanic crust and associated volatile elements, creating new silicic continental crust in volcanic arcs. However, if the mantle was hotter in the Precambrian, the style of subduction, the depth of devolatilization and the formation of silicic continental crust may have been different. Consequently, t...

Mobilization and migration of the heat‐producing elements (HPE) during anatexis is a critical process in the development of orogenic systems, the evolution of continental crust and the stabilization of cratons. In many crustal rocks the accessory minerals are the dominant hosts of Th and U, and the behaviour of these minerals during partial melting...

It is estimated that around three quarters of Earth's first generation continental crust had been produced by the end of the Archaean Eon, 2.5 billion years ago. This ancient continental crust is mostly composed of variably deformed and metamorphosed magmatic rocks of the tonalite–trondhjemite–granodiorite (TTG) suite that formed by partial melting...

The timescales and P-T conditions recorded by granulite facies metamorphic rocks permit inferences about the geodynamic regime in which they formed. Two compositionally heterogeneous cordierite-spinel-bearing granulites from Vizianagaram, Eastern Ghats Province (EGP), India, were investigated to provide P-T-time constraints using petrography, phase...

In this study, we present an example of phase equilibrium modeling of medium-temperature-ultrahigh- pressure (MT-UHP) eclogites that includes consideration of the influence of ferric iron (O) and H2O on the phase equilibria. As a case study, we focus on the intergranular coesite-bearing eclogites at Yangkou in the Sulu Belt. Based on phase equilibr...

On the contemporary Earth, distinct plate tectonic settings are characterized by differences in heat flow that are recorded in metamorphic rocks as differences in apparent thermal gradients. In this study we compile thermal gradients [defined as temperature/pressure (T/P) at the metamorphic peak] and ages of metamorphism (defined as the timing of t...

Coesite is typically found as inclusions in rock-forming or accessory minerals in ultrahigh-pressure (UHP) metamorphic rocks. Thus, the survival of intergranular coesite in UHP eclogite at Yangkou Bay (Sulu belt, eastern China) is surprising and implies locally ‘dry’ conditions throughout exhumation. The dominant structures in the eclogites at Yang...

Geodynamic regimes responsible for the formation of Eoarchean-to-Mesoarchean continental crust may be investigated using numerical modeling in which deeper mantle processes are coupled with shallower processes responsible for the formation and modification of the crust. In an earlier study using a 2D coupled petrological–thermomechanical tectono-ma...

This corrects the article DOI: 10.1038/nature21383.

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high-pressure (HP) mineral assemblage of dominantly quartz+phengite+ allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si-in-phengite barometr...

The geodynamic environment in which Earth’s first continents formed and were stabilized remains controversial1. Most exposed continental crust that can be dated back to the Archaean eon (4 billion to 2.5 billion years ago) comprises tonalite–trondhjemite–granodiorite rocks (TTGs) that were formed through partial melting of hydrated low-magnesium ba...

Anatectic granites from the Fosdick migmatite–granite complex yield U–Pb zircon crystallization ages that range from 115 to 100 Ma, with a dominant grouping at 107–100 Ma, which corresponds to the timing of dome formation during the regional oblique extension that facilitated exhumation of the complex. The occurrence of leucosome-bearing normal-sen...

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Palaeoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts...

Barite inclusions in rock-forming and accessory minerals and in multiphase solid inclusions (MSI) in ultrahigh pressure (UHP) eclogites have been controversially interpreted to record the presence of high-salinity oxidizing fluids In this article we use fluid as a general term for an aqueous fluid phase, a supercritical fluid phase, and a melt phas...

Using petrography, phase equilibria modeling and in-situ (U–Th)–Pb monazite geochronology, we show that pelitic gneiss from close to the bottom of the Qitan1 borehole in the northwest of the Ordos Basin records a four stage metamorphic evolution. The M1–M3 stages, which were suprasolidus, are represented by: rutile relicts in the matrix and biotite...

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist inunderstanding the tectonic style that may have operated in the Archean. Higher mantle tempera-tures and higher radiogenic heat production in the Archean Earth would have impacted the thicknessand composition of the crust. For this reason, well-constrai...

An important question in petrology is whether the production of granite magma in orogens is a closed-system process with respect to mass input from the mantle. This is commonly addressed by inversion of geochemical data from upper crustal granites, but a complementary approach is to assess the kinship of residual granulites and associated granites...

We report geochemical data from (meta-) sedimentary and igneous rocks that crop out in the Ford Ranges of western Marie Byrd Land and discuss the evolution and reworking of the crust in this region during Paleozoic subduction along the former Gondwanan convergent plate margin. Detrital zircon age spectra from the Swanson Formation, a widespread low...

[Cutts et al. (2014)][1] combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure-temperature ( P - T ) data derived from quantitative P - T pseudosections to determine clockwise P - T paths for several samples from the Tjakastad

In this study, in situ U–Pb monazite ages and Lu–Hf garnet geochronology are used to distinguish mineral parageneses developed during Devonian–Carboniferous and Cretaceous events in migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica. SHRIMP U–Pb monazite ages define two dominant populations at 365–30...

In the early 1980s, evidence that crustal rocks had reached temperatures >1000 °C at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic–metasomatic...

Ages retrieved from accessory minerals in high-grade metamorphic rocks place important constraints on the timing of events and the rates of tectonometamorphic processes operating in the deep crust. In suprasolidus rocks, the dissolution and growth of zircon and monazite are strongly dependent on the P–T conditions of metamorphism and the chemistry...

Mantle temperatures during the Archaean eon were higher than today. As a consequence, the primary crust formed at the time is thought to have been extensive, thick and magnesium rich, and underlain by a highly residual mantle. However, the preserved volume of this crust today is low, implying that much of it was recycled back into the mantle. Furth...

The exposed residual crust in the Eastern Ghats Province records ultrahigh temperature (UHT) metamorphic conditions involving extensive crustal anatexis and melt loss. However, there is disagreement about the tectonic evolution of this late Mesoproterozoic–early Neoproterozoic orogen due to conflicting petrological, structural and geochronological...

The thickness and spatial distribution of foliation-parallel leucosomes in metric to decametric scale interlayered units of migmatitic paragneiss and orthogneiss from the Fosdick migmatite-granite complex in West Antarctica are quantified along one-dimensional transects. This study demonstrates that leucosomes in stromatic metatexite migmatites hav...

Cited By (since 1996):5, Export Date: 5 February 2014, Source: Scopus

At low temperatures (<750 degrees C at moderate to high crustal pressures), the production of sufficient melt to reach the melt connectivity transition (similar to 7 vol%), enabling melt drainage, requires an influx of aqueous fluid along structurally controlled pathways or recycling of fluid via migration of melt and exsolution during crystallizat...

The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material fr...

A diorite pluton and widely distributed mafic dykes occur in the Fosdick migmatite–granite complex, which is interpreted to represent middle-to-lower crustal rocks of the paleo-Pacific active continental margin of Gondwana. The mafic dykes exhibit a variety of relationships with host rocks in the field ranging from undeformed dykes with sharp conta...

Petrological data and thermal models suggest ambient upper-mantle potential temperatures (Tp) in the Archean were > or >> 1500 °C, significantly hotter than the present day. Higher ambient Tp would have led to extensive melting of the mantle and the production of a thick (up to 45 km) MgO-rich primary crust that probably formed by over- and intra-a...

There are differences between Archean and post-Archean crust that may be related to Earth's thermal evolution and mechanism of heat loss. The Archean continental crust is dominated by grey gneisses and plutonic complexes of the tonalite-trondhjemite-granodiorite (TTG) suite whereas volcano-sedimentary greenstone belts form only a minor component. T...

The margins of layered igneous intrusions can host ore deposits of nickel, copper, and platinum group elements(PGEs). These marginal deposits are characterized by a complex geologic history, which obscures directevidence for the mineralization process. Mass-independent fractionation of sulfur isotopes is a distinguishingfeature of Archean sedimenta...

Partial melting and melt drainage from deep suprasolidus crust in orogens has important consequences for tectonics. Melt extraction along prograde segments of clockwise P–T paths reduces fertility and increases the density and strength of residual crust, which has implications for further melt production during decompression. Using calculated P–T p...

The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic-ul...

This virtual special issue represents a collection of papers concerning Crustal Melting selected by the Editor from those published on various aspects of this theme in the JMG since 1982. The papers are grouped into sequences that address topics that have been prominent in the JMG during the last 30 years concerning the origin, evolution and tecton...

The Fosdick Mountains in West Antarctica comprise a migmatitic gneiss dome emplaced within a wrench setting during the Cretaceous dextral oblique convergence along the East Gondwana margin that affected both West Antarctica and once-contiguous New Zealand. Three-dimensional exposures offer an expansive views of a region of melt-rich middle crust th...

The Fosdick migmatite–granite complex in West Antarctica records evidence for two high‐temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high‐temperature metamorphic event, both of which involved melting and multiple melt‐loss events, are investigated using phase...

High Mg–Al granulites from the Sunki locality in the central portion of the Eastern Ghats Province record evidence for the high-temperature peak and retrograde evolution. Peak metamorphic phase assemblages from two samples are garnet+orthopyroxene+quartz+ilmenite+melt and orthopyroxene+spinel+sillimanite+melt, respectively. Isochemical phase diagra...

In the southern sector of the Southern Brasília Belt, late Neoproterozoic arc–passive margin collision resulted in juxtaposition of an arc-derived nappe (the Socorro–Guaxupé Nappe) over a stack of passive margin-derived nappes (the Andrelândia Nappe Complex) that lies on top of autochthonous basement of the São Francisco Craton. (U–Th)–Pb monazite...

Partial meting of the continental crust has long been of interest to petrologists as a small-scale phenomenon. Mineral assemblages in the cores of old, eroded mountain chains that formed where continents collided show that the continental crust was buried deeply enough to have melted extensively. Geochemical, experimental, petrological and geodynam...