Premagmatic Zircons Research Articles

Geochronology and petrogenesis of orthogneisses from the Pacov body: implications for the subdivision of the Cambro-Ordovician peraluminous magmatism and related mineralizations in the Monotonous and Varied units of the Moldanubian Zone (Bohemian Massif)

Peraluminous orthogneisses represent a typical rock type of the Monotonous and Varied units of the Moldanubian Zone. The most probable setting for the generation of these rocks is the Cambro-Ordovician magmatic event linked to crustal anataxis, which is related to the thermal relaxation of thickened continental crust. According to their geological position, the mineralogy and geochemistry can be traced to the Blanik and Destna type orthogneisses. The Blanik type is represented by muscovite-biotite to muscovite-tourmaline orthogneisses whose granitic protolith can be generated by muscovite-dominant dehydration melting of metapelites. The chemical heterogeneity of this type suggests magma differentiation, which is manifested by the presence of P-Li-Sn-Nb mineralization. Muscovite-biotite to biotite Destna type orthogneisses were likely generated by the fluid-present melting of immature crustal sources (e.g. metagreywackes and metaigneous rocks) and spatially related to small occurrences of W-bearing mineralization. We applied whole-rock geochemistry combined with mineralogy and zircon U-Pb geochronology to the evolutionary interpretation of one of the most fractionated bodies of Destna type orthogneisses situated near the village of Pacov. The Pacov orthogneiss is accompanied by pre-Variscan greisens and related tungsten mineralization (ferberite, scheelite, pyrite, and native bismuth). The mineral assemblages of these rock types are modified by the Variscan metamorphic overprint (720 ± 75 °C at 0.65 ± 0.29 GPa). Two petrographically distinct samples of orthogneisses give U–Pb zircon ages of 494 ± 2 Ma (garnet-muscovite orthogneiss) and 485 ± 4 Ma (muscovite-biotite orthogneiss). The abundance of inherited pre-magmatic zircon cores (∼614–578 Ma) most likely documents detrital protoliths derived from erosion of the Ediacaran volcanic arc.

Associations between zircon and Fe–Ti oxides in Hiltaba event magmatic rocks, South Australia: atomic- or pluton-scale processes?

The Hiltaba Suite intrusive rocks and penecontemporaneous Gawler Range Volcanics (GRV) comprise the 1590 Ma Gawler silicic large igneous province in the Gawler Craton, South Australia. Zircon is principally associated with Fe–Ti oxides and clusters of touching crystals in these rocks, including in the Roxby Downs Granite (RDG), host of the Olympic Dam iron oxide–copper–gold deposit, and in other intrusive rocks that comprise the Olympic Province. There has been no explicit evaluation and explanation of potential origins published for concentrations of zircon with Fe–Ti oxides (herein zircon-rich clusters) found in these and similar rocks of western North America and elsewhere. Here we use U–Pb geochronology, mineral morphologies and compositions, and insights from surface chemistry and liquid-bound particle interaction studies to investigate zircon-rich clusters and provide a model for their formation. U–Pb geochronology does not reveal any concordant zircon populations older than ca 1590 Ma, so it is unlikely that there are significant xenocrystic zircon grains or that the zircons include significant inherited cores. The lack of pre-magmatic zircon, consistent intra-grain and inter-grain zircon compositional trends, the predominance of oscillatory zoned zircon with morphologies indicating growth from hot, evolved silicate melts, and the lack of evidence for zircon recrystallisation, indicates that zircon crystallised in the host GRV and RDG magmas. Variable zircon compositions within individual clusters does not support epitaxial nucleation of zircon on Fe–Ti oxides, but it is likely that some zircon grains grew from seed crystals formed by exsolution of Zr from Fe–Ti oxides. Aggregation of isolated, liquid-bound crystals is energetically favourable, and the grainsize discrepancy between larger crystals (Fe–Ti oxides, pyroxenes) and smaller accessory minerals (zircon, apatite) maximises the disparity in particle velocities and hence enhances the opportunities for collisions and adhesion between these crystals. We propose that zircon adheres to Fe–Ti oxides with greater ease and/or with greater bond strengths, than to other phases present in the parental magmas. It is possible that this association is related to interactions between zircon and Fe–Ti oxide surface sites with opposing charges, presuming the distance between phase surfaces is sufficiently small. The occurrence of small zircon grains within Fe–Ti oxides and both euhedral zircon and zircon with asymmetric growth zonation in contact with Fe–Ti oxides indicates that several processes are responsible for the high concentrations of zircon crystals in some Fe–Ti oxide clusters.Zircon is principally associated with Fe--Ti oxides in 1.59 Ga Gawler Range Volcanics (GRV) and Roxby Downs Granite (RDG)U–Pb geochronology does not reveal any concordant zircon populations older than ca 1590 MaZircon compositions and morphologies indicate that zircon crystallised in the host RDG and GRV magmas and suggest recharge, reheating and mixing occurred in these magmatic systemsSeed crystals, aggregation and surface chemical affinities contributed to the strong association of zircon and Fe–Ti oxides

Juxtaposition of allochthonous terranes in the central Korean Peninsula: Evidence from zircon U-Pb ages and O-Hf isotopes in Jurassic granitoids

The eventual docking of an exotic continental mass over the course of a Wilson cycle is marked by intracontinental sites of prograde metamorphism, contractional deformation, and syn- or post-collisional magmatism. This study shows that inherited premagmatic zircon in subsequently emplaced granitoid rocks provides another line of evidence for the collisional orogeny. Following the continental collision between the North and South China Cratons, Jurassic granitoids intruded widely throughout the central Korean Peninsula in association with the subduction of the Paleo-Pacific plate. The latest Early to Middle Jurassic (177–167Ma) granitoids occur throughout the cratonic Gyeonggi Massif, whereas Early Jurassic (194–184Ma) granitoids occur locally along the Okcheon Metamorphic Belt to the southeast of the massif. These two spatiotemporal groups differ profoundly in their age patterns and O-Hf isotopic signatures of inherited zircon. The younger group yields a typically North China-like Archean-Paleoproterozoic (mostly 2.5–1.8Ga) zircon inheritance. In contrast, inherited zircons from the older group exhibit exclusively Neoproterozoic (ca. 0.9–0.7Ga) and Paleozoic (ca. 0.4Ga) cluster along with Archean-Paleoproterozoic populations. The exotic feature of the protolith of the older group is further indicated by significantly lower δ18O (−0.9 to +4.9‰) and positive εHf (+3 to +15) values of the Cryogenian zircon cores. The results of zircon analysis demonstrate that South and North China-like terranes were juxtaposed infracrustally along the southeastern margin of the Gyeonggi Massif during the pre-Jurassic collisional orogeny, and then selectively provided major source material to the older and younger Jurassic granitoids, respectively.

Extension-facilitated pulsed S-I-A-type “flare-up” magmatism at 370 Ma along the southeast Gondwana margin in New Zealand: Insights from U-Pb geochronology and geochemistry

New U-Pb zircon geochronology on S-, I-, and A-type granitic plutons in New Zealand refines the magmatic and tectonic history along the mid-Paleozoic Gondwana margin and provides constraints on the episodicity and dynamics of continental arc magmatism under “flare-up” conditions. High-precision isotope-dilution thermal ionization mass spectrometry (ID-TIMS) ages confirm that the voluminous (≥38,500 km3) predominantly S-type granites of the ca. 370–368 Ma Karamea Suite were emplaced within a brief ca. 2.1 Ma window. The Karamea Suite magmatic event represents a short-lived thermal perturbation that resulted in rapid crustal melting and granite emplacement that punctuated the steady-state arc “tempo” of the active Gondwana margin. Recognition of three distinct pulses of silicic magmatism within this ca. 2.1 Ma burst of magmatism, and calculation of flux rates that are comparable to those estimated for Cenozoic ignimbrite “flare-up” events indicate that time scales observed in volcanic complexes at the surface are directly analogous to those observed in plutonic complexes at depth. A genetic association with contemporaneous high-Sr/Y I-type and A-type granitoids suggests a thickened crust immediately prior to, and an extensional setting during Karamea Suite emplacement, respectively. Compositionally, Karamea Suite granites are compatible with an origin by dehydration melting of older infracrustal and supracrustal sources. However, thermal requirements suggest it is unlikely that the overthickened crust was able to reach temperatures hot enough to induce dehydration melting without an additional heat and/or fluid source. Direct involvement of hydrous mantle partial melts (supported by isotopic compositions) and/or an elevated asthenospheric heat flow is therefore advocated as the mechanism that induced extensive melting of the crust. An early postorogenic intra-arc extensional setting is proposed whereby thinning of the crust permitted hot asthenospheric mantle to rise to shallow mantle depths and facilitate melting. Application of laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) has further resolved a distinct zircon inherited (premagmatic) age component within Karamea Suite granites. This inherited age component is interpreted to represent a previously undocumented episode of magmatism along the New Zealand margin of Gondwana at ca. 387 ± 3 Ma, ca. 15 Ma prior to Karamea Suite emplacement. The ca. 387 Ma premagmatic zircon grains were preserved within the ca. 370 Ma Karamea Suite granites as a result of low zircon saturation temperatures ( T Zr < 800 °C). This ca. 387 Ma magmatic episode was volumetrically small, and it may have occurred as a result of Buller-Takaka terrane amalgamation—a crustal thickening event that thermally matured the crust and predisposed it to future melting. Slab rollback and/or delamination of a dense mafic root are suggested as possible triggers that facilitated the rapid change in tectonism from orogenic compression to postorogenic extension. Cessation of voluminous S-type magmatism by ca. 368 Ma was likely caused by depletion of the fertile metasedimentary source rocks that had undergone significant partial melting. Tectonic depletion of the source, by way of stretching the soft, partially melted crust, may also have served to halt production of S-type magmatism, resulting in small-volume production of I- and A-type granites from the less-fertile lower-crustal mafic source rocks.

Timing of Archean crust formation and cratonization in the Awsard-Tichla zone of the NW Reguibat Rise, West African Craton: A SHRIMP, Nd–Sr isotopes, and geochemical reconnaissance study

The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87Sr/86Sr(t) from 0.7003 to 0.7030, ɛNd(t) from +2 to +5, and Nd model ages in the range TCR=3.03–2.92Ga. Eight TTG gneisses yielded SHRIMP U–Pb zircon crystallisation ages between 3.04Ga and 2.92Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a TCR=3.09 and a zircon crystallisation age of 2.94Ga but also contains a 3.11Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03Ga and 3.01Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.

Genetic classification of magmatic rocks from the Alvand plutonic complex, Hamedan, western Iran, based on zircon crystal morphology

The Alvand plutonic complex consists of gabbroic and felsic rocks, the latter can be divided into (1) porphyritic, fine-grained and mylonitic granites and (2) leucocratic granitoids. We investigated the external zircon morphology and their internal structures from all major granitoids of the pluton employing the classic Pupin method supplemented by electron microscope analyses. Zircons of gabbroic rocks are free of visible cores or inclusions and are commonly characterized by {101} pyramids and {100} prisms and show mainly zircon types P5 and D typical for mantel-derived rocks. The zircon population from the porphyritic granite is characterized by the predominance of the pyramidal {211} and prism {110} forms and mainly composed of the subtypes S1, S2, S6 and S7 typical for peraluminous granites of crustal origin. Melt inclusions, recrystallization patches and low-CL intensity rims are typical features in these grains. Zircons from the fine-grained granites are characterized by the predominance of the pyramidal {211} and the prism face {110} and by a preponderance of the subtypes S3, S4, S7 and especially S12 and occasionally S2, L2, L3 and L4, typical for aluminous monzogranites and granodiorites of crustal origin. Some grains have pre-magmatic inherited domains with overgrow rims. The mylonitized granites contain zircons with {101} pyramids and {110} prisms and include subtypes G1, P1, P2, S5 whereas P3, S4, L5 are rarely present, typical for I-type granites. Metamictization, radial cracks and partial overgrowths are prevalent in these zircons. Zircons from the leucocratic granitoids have well-developed magmatic oscillatory zonation and pre-magmatic zircon cores. They are characterized by {101} pyramids and {110} prisms and are mainly composed of subtypes L5, S5, S10 and rarely P1, P2, S2, S3, S4, S7, G1 typical for hybrid calc-alkaline granites.

Diffusion-induced disturbances of the U–Pb isotope system in pre-magmatic zircon and their influence on SIMS dating. A numerical study

Pre-magmatic zircons provide a unique source of information about the nature and origin of magmas, but their original U–Pb ages can be disturbed by the thermal shock caused by entrainment in magma. Disturbances result from the migration of Pb from U-rich to U-poor domains rather than Pb-loss to the surrounding. Our 3D numerical models of Pb diffusion and zircon solution kinetics reveal that pre-magmatic zircons suspended in magma can only be perceptibly disturbed at 800–830°C, but this requires more than 106years. Higher T not only increases the diffusion rate but also exponentially increases the solubility of zircon in the melt so that at T>830–850°C all pre-magmatic zircons in contact with common magmas would be dissolved before diffusion to be effective. Zircons shielded from the melt, on the other hand, can resist much higher temperatures and are surrounded by solid minerals that, in contrast to a melt, do not act as sinks for Pb2+. At T≥1100°C these zircons require very short times to develop large disturbances, as illustrated in xenolithic zircons from a lamproite from SE Spain. By contrast, at 1000°C even moderate disturbances require 104 to 107years, depending on the inter-domain concentration ratio. Accordingly, the diffusive redistribution of Pb in pre-magmatic zircons is most likely limited to two natural scenarios (i) crustal magmas derived from long-lived anatectic complexes, and (ii) crust-contaminated mafic magmas in which zircons survived shielded as inclusions in primocrysts or within xenoliths. Diffusion-induced disturbances tend to yield higher-than-original U–Pb ages that are extremely difficult to detect as illustrated in an autochthonous leucosome from NW Spain, in which the age of pre-magmatic zircon cores fails to reflect the age of the spatially-related source. The interpretation of SIMS U–Pb data for pre-magmatic zircons is not straightforward, so the resulting ages should be considered carefully.

Zircon geochronology, elemental and Sr-Nd isotope geochemistry of two Variscan granitoids from the Odenwald-Spessart crystalline complex (mid-German crystalline rise)

The crystalline parts of the Bergstrasser (western) Odenwald and the southern Spessart expose Variscan I-type granitoids of the mid-German crystalline rise that formed during subduction of the Rheic ocean and collision of Avalonia and Armorica about 365 and 330 Ma ago. We present geochemical, Sr-Nd isotopic, single zircon 207Pb/206Pb evaporation and conventional U-Pb data from a diorite-granodiorite complex of the southern Spessart and from a flasergranitoid of the Bergstrasser Odenwald unit II. Both intrusions provide almost identical zircon ages (332.4 ± 1.6 Ma for Odenwald and 330.4 ± 2.0 Ma for Spessart). Lack of inherited or pre-magmatic zircon components connotes magma genesis in deep crustal hot zones despite low temperature estimates (758–786 °C) derived from zircon saturation thermometry. Investigated rock samples display normal- to high-K calc-alkaline metaluminous (Spessart) and weakly peraluminous (Odenwald) geochemical characteristics. The Spessart pluton has lower eNd(T) values (−2.3 to −3.0) and higher 87Sr/86Sri ratios (0.7060 to 0.7066) compared to Odenwald flasergranitoid (eNd(T) = −0.8 and 87Sr/86Sri = 0.7048). In terms of the tectonic setting, the diorite-granodiorite complex of the southern Spessart forms the continuation of the north Armorican arc segment exposed in the Bergstrasser Odenwald. Taking into account previously reported geochemical and isotopic results, it is concluded that the Spessart pluton does not match compositions of Odenwald unit II granitoids but likely represents the north-eastward extension of unit III.

Coupled elemental and isotopic analyses of polygenetic zircons from granitic rocks by ion microprobe, with implications for melt evolution and the sources of granitic magmas

Sequential Pb isotopic and trace element analyses were applied to the geochemical characterization of polygenetic zircons in the San Bernardino and Granite Mountains intrusive suites, two contemporaneous but compositionally distinct suites emplaced in the early evolutionary stage of the western U.S. Cordillera magmatic arc. These coupled analyses allow characterization of both melt evolution and regional diversity in the principal rock types present in the crustal sources of these two suites. Zircon grains in both intrusive suites exhibit consistent compositional zonation — Hf solid solution in zircon generally increases with cooling and crystallization, and with other indices of melt evolution, including total REE abundances and especially the MREE as indicated by increasing Yb/Gd ratios. These temperature-dependent fractionations are superimposed on persistent parental melt compositional differences in Hf/Zr and Yb/Gd ratios. Premagmatic zircons of Paleoproterozoic to Mesoproterozoic age are common across multiple samples from each suite, and their age and compositional variations image the crustal sources of these magmas. The high K San Bernardino suite had a lithologically comparatively simple 1.7 Ga metaigneous source, whereas the Granite Mountains suite had a source distinguished by the presence of hotter, 1.7 Ga metaigneous rocks hybridized by later intrusion of 1.4 Ga rocks. The absence of evidence for Grenville-age or Paleozoic-age zircons severely limits input in these intrusive suites from either upper crustal sedimentary rocks at the level of emplacement or underthrust sediments derived from erosion of the continental margin.

The ∼844 Ma Moneiga quartz-diorites of the Sinai, Egypt: Evidence for Andean-type arc or rift-related magmatism in the Arabian-Nubian Shield?

U-Pb ion-microprobe zircon dating of the Moneiga quartz-diorites, located within the Katherine ring-complex of the Sinai, reveals a crystallization age of 844 ± 4 Ma and the presence of 1045 Ma pre-magmatic zircon cores. These rocks are much older than previously believed, and represent the oldest magmatic rocks so far dated in the Sinai. Their chemical composition corresponds to mildly silicic, low- to medium-K, metaluminous, calcic to calc-alkaline rocks with somewhat elevated FeO total/MgO, low Ba/La and high Cs/Sc and Zr/Yb. The initial Sr and Nd isotope composition ( 86Sr/ 87Sr 844Ma = 0.703187 ± 0.000034 and ɛNd 844Ma 3.5 ± 0.5) is slightly less primitive than expected from similar rocks of Arabian-Nubian Shield with the same age. Trace-element and isotope ratios reveal the presence of a non-juvenile crustal component that caused a T DM = 1.06 ± 0.06 Ga identical to the age of the oldest pre-magmatic cores of zircons. These data support the idea that the Moneiga quartz-diorites formed by re-melting of a ∼200 Ma old island-arc crust during the early stages of the prolonged break-up process of Rodinia.

Assembling and Disassembling California: A Zircon and Monazite Geochronologic Framework for Proterozoic Crustal Evolution in Southern California

Abstract The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz monzodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790–1730 and 1690–1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900–1790 Ma Paleoproterozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711–1689 and 1670–1650 Ma. Structures record early, tight to isoclinal folding and penetrative west‐vergent shear during the final metamorphic event in the west Mojave province. Prot...

Paleoproterozoic crust within the Great Falls tectonic zone: Implications for the assembly of southern Laurentia

The Great Falls tectonic zone and the Vulcan structure both have been proposed as the site of a Paleoproterozoic suture between the Archean Hearne and Wyoming provinces. Both hypotheses remain viable because all Precambrian rocks composing the Vulcan structure and much of the Great Falls tectonic zone are buried beneath Phanerozoic cover. The primary exceptions to this are the mafic to felsic igneous and metamorphic rocks of the Little Belt Mountains (Montana), previously considered the northernmost exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite (207Pb/206Pb age: 1864 ± 5 Ma) and a gneissic unit intruded by the Pinto (207Pb/206Pb age: 1867 ± 6 Ma), however, confirm their Paleoproterozoic age. These rocks exhibit an overall calc-alkaline affinity and the depletion in high field strength elements typical of convergent margin environments. Whole-rock Sm-Nd data (initial epsilon of −1 to +4) and a lack of premagmatic zircons indicate that the magmas were principally derived from a depleted mantle source, not from older crust. These data suggest that at least some rocks within the Great Falls tectonic zone originated at a convergent margin that developed during the closure of an ocean basin along the northwestern margin of the Wyoming craton ca. 1.9 Ga.

Contrasts between SmNd whole-rock and UPb zircon systematics in the Tobacco Root batholith, Montana: implications for the determination of crustal age provinces

Proper documentation of the extent and age of crust in the western US is critical for constraining a variety of geologic problems ranging from the growth rate of continents to Precambrian continental reconstructions. The secondary isotopic systematics of granitoids have been one of the principal means used to characterize continental crust in areas where the basement is covered. In southwestern Montana and eastern Idaho a group of Late Mesozoic to Cenozoic, dioritic to quartz monzonitic batholiths (e.g., Tobacco Root, Idaho, Pioneer, Boulder, etc.) share a limited range of Paleoproterozoic Sm-Nd depleted mantle model ages. The Tobacco Root batholith (TRB) has a Nd isotopic composition ( ϵ Nd = −17.9 to −19.1) and SmNd model age ( T DM = 1.63 to 1.90 Ga) typical of this group. The TRB, however, intruded Archean crust (∼3.3 Ga, ϵ Nd = ∼ −35), rather than the presumed Proterozoic crust intruded by the other plutons. The Archean heritage of the TRB is confirmed by the presence of premagmatic zircons which range from 2.2 to 3.0 Ga. The combination of U-Pb zircon and Nd model ages suggest that the batholith was derived from both Archean and Proterozoic crustal sources, as well as an ∼80 Ma mantle component. This contrasts with a sample from the northern Idaho batholith which exhibits concordancy between its Sm-Nd and premagmatic zircon systems at ∼1.74 Ga. These data point to the difficulties that can occur if crustal age provinces are defined solely on the basis of Nd model ages of younger plutons, particularly in areas such as the northwestern US where Archean and Proterozoic crust is poorly exposed and dispersed over a large area.

Source of the Northeastern Idaho Batholith: Isotopic Evidence for a Paleoproterozoic Terrane in the Northwestern U.S.

The northeastern portion of the Idaho batholith (NIB) intruded Proterozoic rocks of the Belt-Purcell supergroup between 50 and 90 Ma. Whole-rock Sm-Nd isotopic analyses of batholithic rocks yield depleted mantle model ages ($$T_{DM}$$) between 1.72 and 1.93 Ga and values of $$\epsilon_{Nd}$$ between -17.7 and -21.2, similar to associated metamorphic rocks and within the range for Belt-Purcell sedimentary rocks. Premagmatic zircons from one sample of the NIB were analyzed individually using the SHRIMP ion microprobe and yielded a single age population at 1.74 Ga. This apparently single-aged source contrasts with the range of ages reported for zircons from sedimentary rocks of the Belt-Purcell supergroup and suggests that the batholith was not the product of melting Belt-Purcell sediments, nor was it significantly contaminated with these sediments. The source of the batholith, however, appears to be of appropriate age and composition to be a major contributor of sediment to the Belt basin. In addition, the near coincidence of $$T_{DM}$$ and the age derived from premagmatic zircons in one sample suggests the source of at least part of the batholith was extracted largely from 1.74 Ga depleted mantle, with little or no input from older rocks. If so, this crust may represent a possible continuation of crust of similar age and character exposed to the north in the Canadian cordillera and to the south in Nevada, Arizona, and southeastern California.