Granitic batholiths of the ∼500 Ma Ross Orogen in Antarctica are voluminous in scale, reflecting prolific magmatism along the active early Paleozoic convergent margin of Gondwana. New age and isotopic analysis of zircons from a large suite of Ross granitoids spanning >2,000 km along the orogen provide a wealth of geochronologic, tracer, and inheritance information, enabling us to investigate the pace of magmatism, along‐strike temporal and geochemical trends, magmatic sources, and tectonic modes of convergence. Because granitoids penetrate the crust of the earlier Neoproterozoic rift margin, they also provide insight into the age and composition of the largely ice‐covered East Antarctic craton. Zircon U‐Pb ages from these and other samples indicate that active Ross magmatism spanned 475–590 Ma, much longer than generally regarded. Most samples have heavy zircon δ18O values between 6.5 and 11.5‰ and initial εHfcompositions between 0 and −15; their isotopic co‐variations are independent of age, as in other contemporary continental arcs, and reflect largely crustal melt sources. Samples near Shackleton Glacier have distinctly more mantle‐like isotope composition (i.e., radiogenic εHfand low δ18O) and separate two regions with distinctive isotopic properties and inheritance patterns—a more juvenile section of Mesoproterozoic crust underlying the southern TAM and an older, more evolved region of Paleoproterozoic and Archean crust in the central TAM. The isotopic discontinuity separating these regions indicates the presence of a cryptic crustal boundary of Grenvillian or younger age within the East Antarctic shield that may be traceable into the western Laurentian part of the Rodinia supercontinent.
The Montana metasedimentary terrane in the northern Wyoming Province provides valuable insight into crustal formation and reworking processes along the cratonic margin and offers a unique opportunity to decipher the complex Neoarchean−Paleoproterozoic terrane assembly in southwestern Laurentia. We report new zircon U-Pb dates and Hf isotopes from seven metaigneous samples in the northwestern Montana metasedimentary terrane. The internal textures of zircon in this study are complex; some lack inherited cores and metamorphic overgrowths, while others exhibit core-rim relationships. Based on the cathodoluminescence (CL) features, we interpret these grains to be magmatic populations. These data demonstrate discrete igneous pulses at 2.7 Ga, 2.4 Ga, and 1.7 Ga, which indicate significant crustal formation intervals in the Montana metasedimentary terrane. Zircons at 2.7 Ga have positive εHf values (+2.4 to +0.9) that indicate a depleted mantle source. Most 2.4 Ga and 1.7 Ga samples have negative εHf values (−1.6 to −15.5), which indicate significant contributions from preexisting crust. Two 1.7 Ga samples, however, have near-chondritic εHf values (+0.4 to +0.3) that indicate larger juvenile contributions. The time-integrated Hf isotope trend suggests that the Paleoproterozoic zircons were produced from a mixture of older crust and juvenile mantle inputs. Additionally, the isotopic age fingerprint of the Montana metasedimentary terrane suggests that it differs from northern-bounding terranes. Viewed more broadly, the 2.7 Ga and 1.7 Ga age peaks that the Montana metasedimentary terrane shares with the global zircon age spectrum suggest that the drivers of these events in the Montana metasedimentary terrane were common throughout the Earth and may be associated with the assembly of supercontinents Kenorland and Nuna.
more » « less- Award ID(s):
- 1854390
- NSF-PAR ID:
- 10529389
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The timing of assembly and tectonic origins of terranes in the northern Cordillera of Alaska, British Columbia, and the Pacific Northwest are debated. Stikinia, a long‐lived arc terrane, has an enigmatic regional Mesozoic accretionary history and its tectonic origins remain unconstrained. Zircon U‐Pb geochronology and Lu‐Hf isotopic data on Triassic–Jurassic sedimentary and igneous rocks from central Stikinia shed light on the terrane‐scale effects of a latest Triassic–Early Jurassic collision between Stikinia and pericratonic Yukon‐Tanana terrane. Main age peaks from central Stikinia are 250–160 Ma, reflecting ongoing Mesozoic arc‐related igneous activity within Stikinia. Comparison of isotopic evolution and unconformity development between central Stikinia and northern Stikinia (Whitehorse trough) provide new constraints on regional latest Triassic–earliest Jurassic deformation. We attribute the shortening‐related deformation to variable along‐strike interactions during end‐on collision with the Yukon‐Tanana terrane, with significant crustal thickening at the northern apex of Stikinia that did not persist farther south. A small pre‐Devonian zircon population is significant, as the oldest exposed rocks in Stikinia are Early Devonian. Pre‐Devonian age peaks differ from those of the northern Yukon‐Tanana terrane, but resemble zircons from southern Wrangellia. These zircons are likely multi‐cyclic, derived from crust that originated in the Arctic region near the northern end of the Caledonide orogeny. We suggest that Stikinia was an independent crustal block prior to latest Triassic onset of collision with Yukon‐Tanana terrane. The ongoing, end‐on collision in turn promoted oroclinal assembly of the peri‐Laurentian terranes.
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null (Ed.)Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America.more » « less
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Abstract The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have18O-enriched values (δ18O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ18O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ18O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/B37160.1
