More Like this

1. New geochemical and geochronological insights on forearc magmatism across the Sanak-Baranof belt, southern Alaska: A tale of two belts

   https://doi.org/10.1130/GES02642.1  

   Wackett, Adrian A; Smith, Diane R; Davidson, Cameron; Garver, John I (March 2024, Geosphere)

   Abstract The Sanak-Baranof belt includes a series of near-trench plutons that intrude the outboard Chugach–Prince William terrane over ~2200 km along the southern Alaskan margin. We present new petrological, geochronological, and geochemical data for comagmatic microgranitoid enclaves and granitoid rocks from the Crawfish Inlet (ca. 53–47 Ma) and Krestof Island (ca. 52 Ma) plutons on Baranof and Krestof Islands, as well as the Mount Stamy (ca. 51 Ma) and Mount Draper (ca. 54–53 Ma) plutons and associated mafic rocks that intrude the Boundary block at Nunatak Fiord near Yakutat, Alaska, USA. These data suggest that intrusion of the Sanak-Baranof belt plutons is actually a tale of two distinct belts: a western belt with crystallization ages that young systematically from west to east (63–56 Ma) and an eastern belt with crystallization ages ranging from 55 to 47 Ma, but with no clear age progression along the margin. Hf isotope analyses of magmatic zircon from the western Sanak-Baranof belt become increasingly evolved toward the east with εHft = 9.3 ± 0.7 on Sanak Island versus εHft = 5.1 ± 0.5 for the Hive Island pluton in Resurrection Bay. The Hf isotope ratios of eastern Sanak-Baranof belt rocks also vary systematically with age but in reverse, with more evolved ratios in the oldest plutons (εHft = +4.7 ± 0.7) and more primitive ratios in the youngest plutons (εHft = +13.7 ± 0.7). We propose that these findings indicate distinct modes of origin and emplacement histories for the western and eastern segments of the Sanak-Baranof belt, and that the petrogenesis of eastern Sanak-Baranof belt plutons (emplaced subsequent to 57–55 Ma) was associated with an increasing mantle component supplied to the youngest eastern Sanak-Baranof belt magmas. These plutons reveal important information about offshore plate geometries and a dynamic period of plate reorganization ca. 57–55 Ma, but a clearer picture of the tectonic setting that facilitated these Sanak-Baranof belt intrusions cannot be resolved until the magnitude and significance of lateral translation of the Chugach–Prince William terrane are better understood. 

   more » « less
2. Age of Skarn Formation and Meteoric Water Infiltration in the Willsboro–Lewis Wollastonite District (Adirondack Highlands, New York)

   https://doi.org/10.1093/petrology/egaf024  

   Peck, William H; Regan, Sean P; Blum, Tyler B; Valley, John W; Timothy, Samuel C (April 2025, Journal of Petrology)

   Abstract The Willsboro–Lewis wollastonite district occurs along the margin of the 1.15-Ga Marcy anorthosite massif in the Adirondack Highlands (New York) and records mineralogical and isotopic evidence for formation in the anorthosite’s low-pressure metamorphic contact aureole. Wollastonite–garnet–pyroxene gneisses in the ~25-km-long, 1.5-km-thick skarn belt are mined for wollastonite and are intercalated with massive garnetite and pyroxene ± garnet skarns, all of which have low oxygen isotope ratios indicating circulation of heated meteoric water and relatively shallow depths above the brittle–ductile transition during their formation. Anorthosite, skarns, and country rocks were all variably deformed and recrystallized at depths of 25 to 30 km during the 1.09- to 1.02-Ga Ottawan phase, and locally altered during the 1.01- to 0.98-Ga Rigolet phase, of the Grenvillian orogeny. This study examined rare zircon in low-δ18O skarn rocks to constrain the timing of surface-derived meteoric water infiltration. Zircon was dated, and trace elements were measured by laser-ablation ICPMS, and oxygen isotopes were measured by ion microprobe, yielding a spectrum of ages and oxygen isotope ratios reflecting the polymetamorphic history of these rocks. Most samples are dominated by metamorphic zircon having Ottawan or Rigolet 207Pb/206Pb ages and are in high-temperature oxygen isotopic equilibrium with host wollastonite, garnet and/or pyroxene. Several samples contain igneous zircon with disturbed U–Pb isotope systematics, reflecting some combination of new zircon growth and recrystallization during subsequent metamorphism. Relict 1150–1140 Ma ages are preserved in some zircon cores, which are taken as the ages of igneous zircon incorporated during skarn formation or from protoliths. Some of these 1150 to 1140 Ma cores preserve the low-δ18O record of interaction with meteoric water. Ages seen in the Willsboro–Lewis skarns reproduce the span of igneous, disturbed and metamorphic ages in Adirondack anorthosite, and point to contemporaneous anorthosite emplacement, meteoric water circulation and skarn formation at ca. 1150 Ma. This result is consistent with shallow emplacement of the Marcy anorthosite massif during crustal thinning related to the collapse of the 1.19- to 1.14-Ga Shawinigan orogeny, and that granulite facies overprinting was a later tectonic event. 

   more » « less
3. P–T –t Path of Unusual Garnet–Kyanite–Staurolite–Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

   https://doi.org/10.1093/petrology/egac068  

   Kośmińska, Karolina; Gilotti, Jane A; McClelland, William C; Coble, Matthew A; Thomas, Jay B (August 2022, Journal of Petrology)

   Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion. 

   more » « less
4. Trace elements in zircon record changing magmatic processes and the multi-stage build-up of Archean proto-continental crust

   https://doi.org/10.1016/j.gca.2024.03.014  

   Drabon, Nadja; Kirkpatrick, Heather M; Byerly, Gary R; Wooden, Joseph L (May 2024, Geochimica et Cosmochimica Acta)

   Iizuka, Tsuyoshi (Ed.)

   Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This has been especially important in early-Earth settings where most of the crust has been lost, but in some fortuitous instances detrital zircons derived from that lost crust have been preserved in younger sediments. To study the formation and geochemical evolution of continental crust from the Hadean to the Paleoarchean, the 3.6 to 3.2 Ga Barberton Greenstone Belt in southern Africa is an excellent target due to its outstanding preservation and presence of detrital zircons that span almost a billion years. Here, we use trace elements, in combination with hafnium and oxygen isotopes, of 3.65 to 3.22 Ga detrital and tuffaceous zircons of the Moodies and Fig Tree groups and compare their geochemistry to previously studied 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed of the Onverwacht Group. The major detrital zircon age clusters in the former at 3.55 Ga, 3.46 Ga, and 3.26–3.23 Ga overlap with episodes of TTG emplacement and felsic volcanism in the Barberton area, suggesting a local provenance. In contrast, age clusters at 3.65 Ga and 3.29 Ga of the Moodies and Fig Tree groups as well as 4.2 to 3.3 Ga detrital zircons from the Green Sandstone Bed do not have known intrusive sources and were likely derived from outside the present-day Barberton belt. This indicates that more than half of the felsic igneous events in the detrital zircon record do not have a whole-rock representation that can be directly studied. The similar compositions and inferred crustal evolution histories recorded in zircons from the Fig Tree and Moodies groups, as well as from the Green Sandstone Bed, suggest that they were derived from connected terranes experiencing similar crustal processes diachronously. Together, they show three phases of felsic continent formation, reflecting different crustal processes: (1) long-lived protocrust formed in the Hadean from undepleted mantle sources. These zircons are vastly different from younger zircons and, hence, Barberton TTGs are not good analogues of Hadean crust formation. (2) At 3.8 Ga, onset of significant crustal growth though cyclic juvenile additions and hydrous melting, possibly within a volcanic plateau setting but an arc-like setting cannot be excluded based on this data. (3) Between 3.4 and 3.3 Ga, felsic crust is generated through a previously unrecognized episode of crustal growth by shallow melting of mafic, mantle-derived sources. This is immediately followed by the onset of crustal thickening through the transport of surface-altered, hydrated materials to deep crustal levels. Since there is geological evidence for extension and shortening at that time this may reflect the onset of horizontal movement. Whether this last geodynamic setting reflects modern-style plate tectonics or not, continent formation and the onset of plate tectonics in the Barberton area occurred through complex multi-stage processes spanning almost a billion years, most of which is only accessible through the detrital zircon record. 

   more » « less
5. Early Paleogene Magmatism in the Pinaleño Mountains, Arizona: Evidence for Crustal Melting of Diverse Basement Assemblages during the Laramide Orogeny

   https://doi.org/10.1093/petrology/egab095  

   Scoggin, Shane H; Chapman, James B; Shields, Jessie E; Trzinski, Adam E; Ducea, Mihai N (December 2021, Journal of Petrology)

   Granitic rocks, interpreted to be related to crustal melting, were emplaced into regions of thickened crust in southern Arizona during the Laramide orogeny (80–40 Ma). Laramide-age anatectic rocks are exposed as plutons, sills, and dike networks that are commonly found in the exhumed footwalls of metamorphic core complexes. This study investigates newly discovered exposures of granodioritic–leucogranitic rocks from three intrusive phases in the footwall of the Pinaleño–Jackson Mountain metamorphic core complex of southeastern Arizona, called the Relleno suite. Zircon U–Pb geochronology indicates that the suite was emplaced from 58 to 52 Ma. Zircon Lu/Hf isotope geochemistry, whole-rock Sr and Nd isotope geochemistry, and mineral O isotope geochemistry were used to investigate the source of these rocks and evaluate whether they are related to crustal anatexis. Average zircon εHf(t) values of the suite range from −4.7 to −7.9, whole-rock εNd(i) and 87Sr/86Sr(i) values range from −9.4 to −11.8 and 0.7064 to 0.7094 respectively, and quartz δ18OVSMOW values range from 6.8 to 9.4 ‰. Isotopic and geochemical data of these rocks are consistent with derivation from and assimilation of intermediate–mafic (meta)igneous rocks, at deep crustal levels, and are supported by thermodynamic melt models of Proterozoic igneous rocks equivalent to those exposed in the Pinaleño Mountains. In comparison with other Laramide-age anatectic granites in SE Arizona, those exposed in the Pinaleño Mountains are temporally similar but present compositional and isotopic differences that reflect melting and assimilation of different lithologies, producing distinct mineralogical and isotopic characteristics. The results suggest that crustal melting during this interval was not limited to metasedimentary protoliths and may have affected large portions of the deep crust. The early Paleogene Relleno suite in the Pinaleño Mountains strengthens the relationship between crustal melting and regions of thickened crust associated with the Sevier and Laramide orogenies. 

   more » « less