Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic–Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet–clinopyroxene–phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~ 700 °C and ~ 2.1–2.4 GPa. SHRIMP-RG U–Pb dating of eclogite metamorphic zircon yielded a 75.5 ± 2 Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm–Nd clinopyroxene-two garnet–whole rock isochron from an eclogite band yielded a less precise but consistent age of 77 ± 13 Ma. The above features imply subduction to > 60 km depth of at least a portion of the North American sialic basement during Late Cretaceous collision. The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole + plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5 ± 3.5 Ma SHRIMP-RG U–Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results. Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacús gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua mélange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U–Pb, Rb–Sr, and K–Ar ages of the pegmatite range ~ 76–66 Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary–present strike-slip faulting between the North-American and Caribbean plates

Geochronology and Tectonic Significance of Middle Proterozoic Granitic Orthogneiss, North Qaidam HP/UHP Terrane, Western China

Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U–Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ± Ma and 921 ± 7 Ma. Zircon rims in one of these samples yield younger ages (397–618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ± 18 Ma xenocrystic core and 2.4–2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920–930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism. Included here is the post-print copy of this article. The final publication is available at Springer via http://dx.doi.org/10.1007/s00710-006-0149-1

Epidote Minerals in High P/T Metamorphic Terranes: Subduction Zone and High- to Ultrahigh-pressure Metamorphism

This chapter focuses mainly on modes of occurrences of zoisite, clinozoisite-epidote and piemontite in various rock types from selected subduction zones and HP to UHP metamorphic terranes. The amount of information on these rocks has increased dramatically during the last 20 years and it is virtually impossible to completely review all. We therefore restrict this review mainly to those localities where we have our own experience, and add some information from other localities. Epidote minerals in metamorphic rocks that followed classical barrovian- or Buchan-type metamorphism are reviewed by Grapes and Hoskin (2004). Nevertheless, as the P-T conditions encountered during subduction zone and Barrovian-type metamorphism are not thus distinct in the P-T range of the greenschist, blueschist, to epidote-amphibolite facies transition, some overlaps between Grapes and Hoskin (2004) and this chapter were found to be unavoidable. As subduction zone metamorphism represents also a potential prograde path of many HP and UHP rocks we will first review some selected and well studied examples of subduction zone metamorphism and then prograde into the field of the peculiar HP and UHP metamorphic rocks. Specifically, this chapter deals with the following major subjects: (1) Prograde sequences from lawsonite to epidote zone blueschists and the corresponding epidote-forming reactions are described from the Franciscan, New Caledonian and Sanbagawa metamorphic rocks; these examples are used to illustrate the role of epidote minerals in typical subduction zone metamorphic successions. (2) Phase relations of zoisite-bearing assemblages in calcareous metasediments are briefly reviewed for eclogite facies rocks in the European Alps. (3) The control of bulk rock composition on paragenesis and compositions of UHP phases is exemplified by various zoisite-bearing UHP eclogites from Dabie (central China) with an AFM diagram. (4) The common break down of plagioclase in HP and UHP rocks in the presence of H2O to form the assemblage zoisite + kyanite + sodic clinopyroxene + quartz is illustrated in some HP to UHP coronitic metagranites and metagabbros. (5) The common but selective occurrence of epidote minerals in Sulu-Dabie eclogite, quartzite and gneiss may be related to the oxidation of supracrustal rocks by a fossil hydrothermal system. (6) The pseudomorphic replacements of epidote/zoisite + amphibole + biotite + sodic plagioclase after garnet and of zoisite + albite after kyanite as indicators for a metasomatic and retrograde overprint during exhumation are described using kyanite-phengite-coesite eclogites from Dabie. (7) The coexistence of lawsonite and epidote in some HP metabasites is related to bulk composition, Æ’O2 and metastable persistence. (8) Composition of epidote minerals from buffered assemblages is sensitive to P, T and Æ’O2, and its use as a geobarometer or geothermometer is discussed. (9) Epidote minerals are a key to discuss fluid-rock interaction and partial melting of metamorphic rocks at high-P conditions. (10) Unusually high Sr and REE contents in epidote minerals including allanite suggest that they are the main reservoirs of these elements in subduction zones and HP to UHP environments. (11) Sector-zoning of zoisite and epidote and their petrological and mineralogical significance is also briefly discussed

Epidote-rich Talc-kyanite-phengite Eclogites, Sulu Terrane, Eastern China: P − T − fO\u3csub\u3e2\u3c/sub\u3e Estimates and the Significance of the Epidote-talc Assemblage in Eclogite

Eclogites interlayered with gneiss and minor quartzite in the Qinglongshan near Donghai are characterized by unusually abundant (15–40 vol%) hydrous phases including talc, phengite, and epidote; many also contain kyanite. Garnet hosts both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite; optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed talc. Estimated peak conditions of 30–35 kbar, 600–700◦C, are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si p.f.u. (11 oxygen basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem- Mag buffer, consistent with the epidote + talc assemblage and 5–20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually magnesium-rich bulk compositions. This article is also available via the American Mineralogist at http://www.minsocam.org/msa/ammin/toc/2004/ND04.html

Transition of UHP eclogites to gneissic rocks of low-amphibolite facies during exhumation: evidence from the Dabie terrane, central China

The Shuanghe ultrahigh-pressure (UHP) slab in the Dabie Mountains consists of layered coesite-bearing eclogite, jadeite quartzite, marble and biotite gneiss, and is fault bounded against hosting orthogneiss. Representative assemblages of eclogite are Grt+Omp+Coe+Rt±Ky±Phn±Mgs; it formed at P>27 kbar and 680–720±50 °C. During exhumation, these UHP rocks experienced multistage retrograde metamorphism. Coesite was overprinted by quartz aggregates, phengite by biotite±muscovite and rutile by titanite. Garnet was successively replaced by a thin rim of Amp, Amp+Pl, and Amp+Ep±Bt+Pl (minor). Omphacite and kyanite were replaced by Amp+Pl±Cpx (or ±Bt) and by Zo+Pl+Ms±Mrg±Bt, respectively. Secondary calcite occurs as irregular pockets in some layers. An outcrop near the UHP slab border is composed of not, vert, similar20 thin, concordant layers of foliated eclogites, amphibolite and gneissic rocks of variable bulk composition. These layers exhibit mineral assemblages and textures transitional from less altered through extensively retrograded eclogite to gneissic rock of low-amphibolite facies through hydration, metasomatism and recrystallization. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Retrograde metamorphism of variable extent may be attributed to selective infiltration of retrograde fluids of CO2-rich and low-salinity aqueous, intensity of deformation and mineral resistance to alteration. The fluid phase for retrogression may have occurred either as discontinuous flow along grain boundaries in completely retrograded eclogites, and/or as isolated pockets in extensive or less altered eclogite layers

Age and Duration of Eclogite-facies Metamorphism, North Qaidam HP/UHP Terrane, Western China

Amphibolite-facies para- and orthogneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor eclogite and peridotite which record ultra-high pressure (UHP) metamorphism associated with the Early Paleozoic continental collision of the Qilian and Qaidam microplates. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb and REE analyses of zircons from four eclogites yield weighted mean ages of 449–422 Ma, and REE patterns (flat HREE, no Eu anomaly) and inclusions of garnet, omphacite, and rutile indicate these ages record eclogite-facies metamorphism. The coherent field relations of these samples, and the similar range of individual ages in each sample suggests that the ∼25 m.y. age range reflects the duration of eclogite-facies conditions in the studied samples. Analyses from zircon cores in one sample yield scattered 433–474 Ma ages, reflecting partial overlap on rims, and constrain the minimum age of eclogite protolith crystallization. Inclusions of Th + REE-rich epidote, and zircon REE patterns are consistent with prograde metamorphic growth. In the L¨uliang Shan, approximately 350 km northwest in the North Qaidam terrane, ages interpreted to record eclogite-facies metamorphism of eclogite and garnet peridotite are as old as 495 Ma and as young as 414 Ma, which suggests that processes responsible for extended high-pressure residence are not restricted to the Dulan region. Evidence of prolonged eclogitefacies metamorphism in HP/UHP localities in the Northeast Greenland eclogite province, the Western Gneiss Region of Norway, and the western Alps suggests that long eclogite-facies residence may be globally significant in continental subduction/collision zones