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1. Assembly and Tectonic Evolution of Continental Lower Crust: Monazite Petrochronology of the Ivrea‐Verbano Zone (Val Strona di Omegna)

   https://doi.org/10.1029/2021TC006841  

   Wyatt, Damaris C.; Smye, Andrew J.; Garber, Joshua M.; Hacker, Bradley R. (March 2022, Tectonics)

   Abstract Metasediments are common constituents of exhumed lower‐to‐mid‐crustal granulite terranes; understanding their emplacement is significant for the assembly and tectonic evolution of deep continental crust. Here, we report a monazite U‐Th‐Pb petrochronological investigation of the Variscan Ivrea‐Verbano Zone (IVZ) (Val Strona di Omegna section)—an archetypal section of lower crust. Monazite Th‐Pb dates from 11 metapelitic samples decrease with structural depth from 310 to 285 Ma for amphibolite‐facies samples to <290 Ma for granulite‐facies samples. These dates exhibit a time‐resolved variation in monazite trace‐element composition, dominated by the effects of plagioclase and garnet partitioning. Monazite growth under prograde to peak metamorphic conditions began as early as 316 ± 2 Ma. Amphibolite‐facies monazite defines a trend consistent with progressively decreasing garnet modal abundances during decompression and cooling starting at ∼310 Ma; the timing of the onset of exhumation decreases to ∼290 Ma at the base of the amphibolite‐facies portion of the section. Structurally lower, granulite‐facies monazite equilibrated under garnet‐present pressure‐temperature conditions at <290 Ma, with monazite (re)crystallization persisting until at least ∼260 Ma. Combined with existing detrital zircon U‐Pb dates, the monazite data define a <30 Myr duration between deposition of clastic sediments and their burial and heating, potentially to peak amphibolite‐to‐granulite‐facies conditions. Similarly brief timescales for deposition, burial and prograde metamorphism of lower crustal sediments have been reported from continental magmatic arc terranes—supporting the interpretation that the IVZ represents sediments accreted to the base of a Variscan arc magmatic system >5 Myr prior to the onset of regional extension and mafic magmatism. 

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2. The temporal evolution of subduction initiation in the Samail ophiolite: High‐precision U–Pb zircon petrochronology of the metamorphic sole

   https://doi.org/10.1111/jmg.12719  

   Rioux, Matthew; Garber, Joshua M.; Searle, Michael; Crowley, James L.; Stevens, Sally; Schmitz, Mark; Kylander‐Clark, Andrew; Leal, Kayla; Ambrose, Tyler; Smye, Andrew J. (May 2023, Journal of Metamorphic Geology)

   Abstract High‐precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well‐preserved and well‐characterized ophiolite, we performed coupled zircon laser‐ablation inductively coupled mass spectrometry trace element analyses and high‐precision isotope dilution‐thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman‐United Arab Emirates). Zircon grains from amphibolite‐ to granulite‐facies (0.8–1.3 GPa, ~700–900°C), garnet‐ and clinopyroxene‐bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet‐clinopyroxene amphibolite samples, Ti‐in‐zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti‐in‐zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphicP–T–tpath. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet‐free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon ε_Hf(+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high‐grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite. 

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3. Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

   https://doi.org/10.2138/am-2022-7967  

   Stowell, H.H. (June 2022, The American mineralogist)

   Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland, New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30 ± 0.13 to 115.70 ± 0.18 Ma (CA-ID-TIMS) and 121.4 ± 2.0 to 109.8 ± 1.8 Ma (SHRIMP-RG). Zircon dates in host and garnet reaction zone do not define distinct populations. In addition, the dates cannot be readily grouped based on external morphology or internal CL zoning. Zircon trace-element concentrations indicate two distinct crystallization trends, clearly seen in Th and U. Garnet occurs in selvages to the leucosome veins and in the adjacent garnet reaction zones. In selvages and host orthogneiss, garnet is generally 0.5 to 1 cm diameter and euhedral and is 0.1 to 0.5 cm diameter and subhedral in garnet reaction zones. Garnet Sm-Nd and Lu-Hf dates range from ca. 115 to 101 Ma (including uncertainties) and correlate with grain size. We interpret the CA-ID-TIMS zircon dates to record the age of magma emplacement and the SHRIMP-RG dates to record a range from igneous crystallization to metamorphic dissolution and reprecipitation and/or local Pb loss. Zircon compositional trends within the garnet reaction zone and host are compatible with locally isolated melt and/or separate intrusive magma batches for the two samples described here. Dates for the largest, ~1 cm, garnet of ~113 Ma record growth during metamorphism, while the smaller grains with younger dates reflect high-temperature intracrystalline diffusion and isotopic closure during cooling. The comprehensive geochronological data set for a single location in the Malaspina Pluton illustrates a complex and protracted geologic history common in granulite facies rocks, estimates lower crustal cooling rates of ~20 °C/m.y., and underlines the importance of multiple chronometers and careful textural characterization for assigning meaningful ages to lower-crustal rocks. Numerous data sets from single locations, like the one described here, are needed to evaluate the spatial extent and variation of cooling rates for Fiordland and other lower crustal exposures. 

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4. Early Jurassic subduction initiation recorded in the Easton metamorphic suite, Northwest Cascades (Washington, USA)

   https://doi.org/10.1130/B38167.1  

   Cunetta, Nicholas S; Mulcahy, Sean R; Schermer, Elizabeth R; Smit, Matthijs A (May 2025, Geological Society of America Bulletin)

   The Easton metamorphic suite is a Mesozoic era subduction complex in northwest and central Washington, USA, which contains amphibolite-facies units structurally overlying separate high- and low-grade blueschist units. New structural, petrographic, and geochronologic data record a complex history related to Early Jurassic subduction initiation. Two types of amphibolite occur as (1) meter-scale coarse garnet amphibolite blocks with an upper amphibolite- to granulite-facies assemblage and (2) continuous layers of ≤10-m-thick foliated garnet amphibolite. The amphibolite blocks are encased in the foliated amphibolite, quartzose schist, and serpentinite. Garnet Lu-Hf geochronology records prograde garnet growth at 203 Ma in the amphibolite blocks and 183 Ma in the foliated amphibolite unit. In situ titanite U-Pb ages on amphibolite blocks, quartzose schist, and foliated amphibolite cluster at 168−163 Ma, with minor inherited components of up to 200 Ma. Amphibole 40Ar/39Ar cooling ages from the amphibolite blocks are 160−158 Ma, which is slightly younger than previously published 40Ar/39Ar cooling ages of 167−165 Ma from the foliated amphibolite. The deformation-temperature-time history of foliated amphibolite records subduction initiation in the Easton metamorphic suite at ca. 183 Ma, followed by cooling to high-grade blueschist facies, ∼500−600 °C, at ca. 165 Ma, and <400 °C by ca. 160 Ma. The 203 Ma coarse amphibolite blocks may have formed in an earlier metamorphic belt before being incorporated into the newly initiated subduction zone at 183 Ma, though an older age of subduction initiation is possible. Combined with existing data from the lower-grade regional blueschists that lie structurally beneath the high-grade rocks, the Easton metamorphic suite preserves >70 m.y. of subduction metamorphism and deformation. Early Jurassic subduction initiation in both the Easton metamorphic suite and the Franciscan Complex of California, USA, reflects broadly synchronous initiation of east-dipping subduction along portions of the Cordilleran margin by 183−176 Ma. 

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5. Metamorphic ages constrain the timing and nature of heat flow into the lower crust of a magmatic arc, Fiordland, New Zealand

   Stowell, H. H.; Schwartz, J.; Klepeis, K.A.; Odom-Parker, K.; Hout, C.; Bollen, E.; Yelverton, J. (December 2017, Transactions - American Geophysical Union)

   Garnet ages for eclogite and granulite from the Western Fiordland Orthogneiss (WFO) provide a precise age for high-grade metamorphism and partial melting of the lower crust in a Cretaceous magmatic arc currently exposed in Fiordland, New Zealand. U/Pb zircon ages and pluton areas indicate that a high magmatic flux event between 118 and 115 Ma added >3,000 km2 of mid- to lower-crustal plutons. The high flux event was followed by high temperature metamorphism and partial melting which resulted in pervasive leucosomes, and trondhjemite layers and veins. At least 1,800 km2 of the newly added crust was metamorphosed to garnet granulite facies orthogneiss. Thermobarometry and phase diagram models indicate that garnet grew at 850 to 1,000°C and 12 to 14 kbar in this monzodiorite and diorite gneiss of the Misty, Malaspina, and Breaksea plutons. Sm-Nd garnet-rock isochrons for these three plutons of the WFO (>700 km2of lower crust) indicate that peak temperatures were reached at 111.7±1.0 Ma (N=16). The isotopic and chemical composition of zircon indicate that the Cretaceous arc flare-up was most likely triggered by partial melting and hybridization of subducted oceanic crust and enriched subcontinental lithospheric mantle directly prior to cessation of arc magmatism. The driving mechanism for the terminal magmatic surge is inferred to be propagation of a discontinuous slab tear beneath the arc, or a ridge-trench collision event between 136 and 128 Ma. The lack of ca. 112 Ma plutons in the western part of Fiordland negates a magmatic heat source for garnet granulite metamorphism. Therefore, we infer that high heat flow associated with mantle advection at the base of the arc after the magmatic surge continued for several m.y., heating the lower crust to granulite facies temperatures 

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