The Application of Laser-Ablation Split-Stream ICP Petrochronology to Metamorphic Studies

Discussion of a new, innovative method for dating rocks, called laser ablation split stream (LASS) petrochronology, which is an in situ method that couples geochronological and geochemical data of minerals that remain in the rock matrix. The talk focuses on the application of this technique with U-Th-Pb dating of the phosphate minerals monazite and xenotine in metamorphic rocks. Examples from the Ruby Range in southwestern Montana and metamorphic core complexes in the northern Idaho panhandle will be explored.

Metamorphic and geochronogical study of the Triassic El Oro metamorphic complex, Ecuador: Implications for high-temperature metamorphism in a forearc zone

In the forearc of the Andean active margin in southwest Ecuador, the El Oro metamorphic complex exhibits a well exposed tilted forearc section partially migmatized. We used Raman spectroscopy on carbonaceous matter (RSCM) thermometry and pseudosections coupled with mineralogical and textural studies to constrain the pressure–temperature (P–T) evolution of the El Oro metamorphic complex during Triassic times. Our results show that anatexis of the continental crust occurred by white-mica and biotite dehydration melting along a 10 km thick crustal domain (from 4.5 to 8 kbar) with increasing temperature from 650 to 700 °C. In the biotite dehydration melting zone, temperature was buffered at 750–820 °C in a 5 km thick layer. The estimated average thermal gradient during peak metamorphism is of 30 °C/km within the migmatitic domain can be partitioned into two apparent gradients parts. The upper part from surface to 7 km depth records a 40–45 °C/km gradient. The lower part records a quasi-adiabatic geotherm with a 10 °C/km gradient consistent with an isothermal melting zone. Migmatites U–Th–Pb geochronology yielded zircon and monazite ages of 229.3 ± 2.1 Ma and 224.5 ± 2.3 Ma, respectively. This thermal event generated S-type magmatism (the Marcabeli granitoid) and was immediately followed by underplating of the high-pressure low-temperature (HP-LT) Arenillas–Panupalí unit at 225.8 ± 1.8 Ma. The association of high-temperature low-pressure (HT-LP) migmatites with HP-LT unit constitutes a new example of a paired metamorphic belt along the South American margin. We propose that in addition to crustal thinning, underplating of the Piedras gabbroic unit before 230 Ma provided the heat source necessary to foster crustal anatexis. Furthermore, its MORB signature shows that the asthenosphere was involved as the source of the heat anomaly. S-type felsic magmatism is widespread during this time and suggests that a large-scale thermal anomaly affected a large part of the South American margin during the late Triassic. We propose that crustal anatexis is related to an anomaly that arose during subduction of the Panthalassa ocean under the South American margin. Slab verticalization or slab break-off can be invoked as the origin of the upwelling of the asthenosphere.

Metamorphism, Transient Mid-Crustal Rheology, Strain Localization and the Exhumation of High-Grade Metamorphic Rocks

We present a series of three-dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid-crustal section in the New England Appalachians. The natural mid-crustal section records evidence for spatially and temporally variable mid-crustal strength as a function of metamorphic grade during prograde polymetamorphism. Our models address changes in strain rate partitioning and topographic uplift as a function of strengthening/weakening in the middle crust, as well as the resultant changes in deformation kinematics and potential exhumation patterns of high-grade metamorphic rock. Results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates. Deformation kinematics recorded in the orogen are also affected by strengthening/weakening, with complete reversals in shear sense occurring as a function of strengthening/weakening without changes in plate boundary kinematics.

Experimental Metamorphic Petrology

Since Bowen and Tuttle's pioneering study of the system MgO-SiO2-H2O in 1949, advances in experimental metamorphic petrology have occurred steadily rather than in “leaps and bounds”. The number and quality of papers published during the past quadrennium, 1975–1978, attests to the health of the science. Although the purpose of this report is to focus international attention on the U.S. effort in experimental metamorphic petrology, some papers published by foreign experimentalists have been included, especially where their contributions complement those made in the U.S. To keep the review current, abstracts of papers read at national meetings of the Geological Society of America and the American Geophysical Union in 1978 are included.

Chevkinite-Group Minerals from Granulite-Facies Metamorphic Rocks and Associated Pegmatites of East Antarctica and South India

Electron microprobe data are presented for chevkinite-group minerals from granulite-facies rocks and associated pegmatities of the Napier Complex and Mawson Station charnockite in East Antarctica and from the Eastern Ghats, South India. Their compositions conform to the general formula for this group, viz. A(4)BC(2)D(2)Si(4)O(22) where, in the analysed specimens A = (rare-earth elements (REE), Ca, Y, Th), B = Fe(2+) Mg, C = (Al, Mg, Ti, Fe(2+), Fe(3+), Zr) and D = Ti and plot within the perrierite field oftlic total Fe (as FeO) (wt.%) vs. CaO (wt.%) discriminator diagram of Macdonald and Belkin (2002). In contrast to most chevkinite-group minerals, the A site shows unusual enrichment in the MREE and HREE relative to the LREE and Ca. In one sample from the Napier Complex, Y is the dominant cation among the total REE + Y in the A site, the first reported case of Y-dominance in the chevkinite group. The minerals include the most Al-rich yet reported in the chevkinite group (<= 9.15 wt.% Al(2)O(3)), sufficient to fill the C site in two samples. Conversely, the amount of Ti in these samples does not fill the D site. and, thus, some of the Al could be making up the deficiency at D, a situation not previously reported in the chevkinite group. Fe abudances are low, requiring Mg to occupy up to 45% of the B site. The chevkinite-group minerals analysed originated from three distinct parageneses: (1) pegmatites containing hornblende and orthopyroxene or garnet; (2) orthopyroxene-bearing gneiss and granulite; (3) highly aluminous paragneisses in which the associated minerals are relatively magnesian or aluminous. Chevkinite-group minerals from the first two parageneses have relatively high FeO content and low MgO and Al(2)O(3) contents; their compositions plot in the field for mafic and intermediate igneous rocks. In contrast, chevkinite-group minerals from the third paragenesis are notably more aluminous and have greater Mg/Fe ratios.

Dating polygenetic metamorphic assemblages along a transect across the Western Alps

Multichronometric analyses were performed on samples from a transect in the French-Italian Western Alps crossing nappes derived from the Briançonnais terrane and the Piemonte-Liguria Ocean, in an endeavour to constrain the high-pressure (HP) metamorphism and the retrogression history. 12 samples of white mica were analysed by 39Ar-40Ar stepwise heating, complemented by 2 samples from the Monte Rosa 100 km to the NE and also attributed to the Briançonnais terrane. One Sm-Nd and three Lu-Hf garnet ages from eclogites were also obtained. White mica ages decrease from ca. 300 Ma in the westernmost samples (Zone Houillère), reaching ca. 300 °C during Alpine metamorphism, to < 48 Ma in the internal units to the East, which reached ca. 500 °C during Alpine orogeny. The conventional “thermochronological” interpretation postulates Cretaceous Eo-Alpine HP metamorphism and younger “cooling ages” in the higher-temperature samples. However, Eocene Lu-Hf and Sm-Nd ages from the same samples cannot be interpreted as post-metamorphic cooling ages, which makes a Cretaceous eclogitization untenable. The age date from this transect require instead to replace conventional “thermochronology” by an approach combining age dating with detailed geochemical, petrological and microstructural investigations. Petrology reveals important mineralogical differences along the transect. Samples from the Zone Houillère mostly contain detrital mica. White mica with Si > 6.45 atoms per formula unit becomes more abundant eastward. Across the whole traverse, HP phengitic mica forms the D1 foliation. Syn-D2 mica is Si-poorer and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist facies conditions. D1 phengite is very often corroded, overgrown or intergrown by syn-D2 muscovite. Most importantly, syn-D2 recrystallization is not limited to S2 schistosity domains; microchemical fingerprinting shows that it also can form pseudomorphs after crystals that could be mistaken to have formed during D1 based on microstructural arguments alone. Thereby the Cl concentration in white mica is a useful discriminator, since D2 retrogression was associated with a less saline fluid than eclogitization. Once the petrological stage is set, geochronology is straightforward. All samples contain mixtures of detrital, syn-D1 and syn-D2 mica, and retrogression phases (D3) in greatly varying proportions according to local pressure-temperature-fluid activity-deformation conditions. The correlation of age vs. Cl/K clearly identifies 47 ± 1 Ma as the age of formation of syn-D1 mica along the entire transect, including the Monte Rosa nappe samples. The inferred age of the greenschist-facies low-Si syn-D2 mica generation ranges within 39-43 Ma, with local variations. Coexistence of D1 and D2 ages, and the constancy of non-reset D1 ages along the entire transect, are strong evidence that the D1 white mica ages are very close to formation ages. Volume diffusion of Ar in white mica (activation energy E = 250 kJ/mol; pressure-adjusted diffusion coefficient D’0 < 0.03 cm2 s-1) has a subordinate effect on mineral ages compared to both prograde and retrograde recrystallization in most samples. Eocene Lu-Hf and Sm-Nd garnet ages are prograde and predate the HP peak.