James M. Mattinson

Lamprophyre dike intrusion and the age of the Alpine fault, New Zealand

The orientations of intrusive rocks from a carbonatitic lamprophyre dike swarm and the history of emplacement relative to country-rock schist structures are compatible with intrusion into tension fractures and Riedel shears formed during initiation of the dextral wrench system of the Alpine fault. New U-Pb and Rb-Sr dates indicate a late Oligocene-early Miocene time of intrusion which, in turn, suggests a mid-Tertiary history for propagation of the Alpine fault plate boundary through South Island, New Zealand.

A study of complex discordance in zircons using step-wise dissolution techniques

Step-wise dissolution techniques applied to a variety of zircon samples, including those with combined inheritance plus later isotopic disturbance, reveal both the complexity of zircon isotopic systematics, and successfully “see through” this complexity to extract high-quality age information. The chemical procedures for the partial dissolution steps must be designed to extract completely all the U and Pb associated with the zircon digested in each step, in order to avoid large, laboratory-induced fluctuations in U/Pb ratio from step to step. In general, relatively short initial partial dissolution steps remove disturbed zircon domains characterized by very high U concentrations and low 206Pb/238U ages. In some cases the initial step yields the lowest 206Pb/238U age, reflecting both the high accessibility and solubility of the most disturbed domains. In other cases, disturbed domains evidently reside deep within the zircons, and are attacked only when the second or third steps penetrate to the interior via cracks or flaws to “mine out” these domains. In all the samples in this study, and regardless of the details of the steps, about a week of digestion time at 80°C removed most of the highly disturbed domains, leaving further partial dissolution steps and/or the total digestion of the final residue to sample highly refractory (i.e., highly insoluble), relatively low U zircon domains. The early partial digestion steps also remove virtually all the common Pb associated with the zircons. Study of partially digested zircons by scanning electron microscope reveals that at least in part, the disturbed, high-U, highly soluble domains and the less (un-?) disturbed, low-U, highly refractory domains are defined by primary igneous zoning on a micron or even sub-micron scale, well below the resolution of the “SHRIMP”, for example. In the case of zircons lacking any inheritance, the residue analyses yield near-concordant, highly precise results. In the case of zircons with inherited components, the residue analyses appear to plot on “primary” discordia lines. Depending on the relative solubility of the inherited versus the igneous component, the final residue will concentrate one or the other.

Uranium‐lead zircon ages from the Median Tectonic Zone, New Zealand

Abstract The Median Tectonic Zone (MTZ) of New Zealand is a generally north trending belt of Mesozoic subduction‐related I‐type plutonic, volcanic, and sedimentary rocks in South Island and Stewart Island that separates Permian strata of the Eastern Province Brook Street Terrane from lower to mid‐Paleozoic Gondwana margin assemblages of the Western Province. High‐precision isotope dilution U/Pb ages of zircons from 30 rocks are reported. Pre‐digestion leaching of zircon in hydrofluoric acid yielded significantly more concordant residues by removing common Pb and dissolving more soluble high‐U domains that have been more affected by relatively recent Pb loss. The results show that MTZ magmatism ranges in age from at least Early Triassic to Early Cretaceous (247–131 Ma), with a pronounced gap in the Middle Jurassic. Triassic plutons tend to occur on the eastern side of the MTZ, and they intrude volcanic/sedimentary sequences of the MTZ in Nelson and eastern Fiordland. These sequences are in turn intruded by...

Age, Origin, and Thermal Histories of Some Plutonic Rocks From the Salinian Block of California

U-Pb isotopic measurements on zircons from some granitic rocks of the Salinian block indicate emplacement and crystallization of these rocks about 104 m.y. ago (mid-Cretaceous). The relatively radiogenic nature of initial Sr and common Pb in these rocks, and the presence of an inherited component of zircon in some of them strongly suggest the involvement of continental crust in the genesis of the magmas. Possibly the magmas were generated in a zone of melting that overlapped the boundary between the mantle and the continental crust.U-Pb measurements on sphene, apatite, and feldspar from the plutons, along with previously published K-Ar and fission-track ages shed light on the post-emplacement thermal histories of the plutons. Most of the samples from the northern part of the Salinian block (Bodega Head, Point Reyes) show relatively simple thermal histories. Evidently these plutons were emplaced at moderate levels in the crust, crystallized, and cooled to moderate temperatures over an interval of about 10–15 m.y. Plutons from the central Salinian block (Santa Lucia Range) show more complex and prolonged thermal histories. These plutons evidently were emplaced at greater depths in the crust than were the plutons from the northern Salinian block. They remained at elevated temperatures for ca. 25 m.y., then cooled fairly rapidly, probably as a result of rapid uplift and erosion. One sample from the northern Salinian block shows an even longer span of time between emplacement and cooling.The thermal evolution of the Salinian plutonic rocks predates major offset along the San Andreas fault zone and thus reflects the thermal evolution of the undisrupted source terrane of the Salinian block. Isotopic measurements of the type reported here might therefore prove valuable in correlations across the San Andreas fault zone. Moreover, detailed study of thermal evolution within the Salinian block could shed light on major offsets within the block.

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism, New Zealand

U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between ∼120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased ≤116 Ma ago based on206Pb/238U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by ∼90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr.Unmetamorphosed gabbronorites of the Darran Complex in eastern Fiordland, inferred by some investigators to be the granulite protolith, yield concordant U/Pb zircon ages of 137±1 Ma. U-Pb ages of apatite, and previously published K/Ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years. The high-grade WFO and unmetamorphosed Darran Complex are now separated by a profound structural break. However, the ages and similarities in initial Pb and Sr isotopic ratios suggest that both suites are products of the same Early Cretaceous cycle of subduction-related magmatism. The timing of Early Cretaceous magmatism and metamorphism, collision and resultant crustal thickening, and subsequent great uplift and erosion in Fiordland has important implications for terrane accretion and the evolution of relative plate motions along the New Zealand segment of the Gondwana margin.

UPb ages of zircons: A basic examination of error propagation

Abstract The past three decades have seen a remarkable development and expansion of the UPb dating technique as applied to zircons. It is regrettable, however, that many otherwise sophisticated zircon studies lack even the most rudimentary error analysis for individual data points, despite the availability of published error programs, while others use such programs incorrectly, and report highly unrealistic error estimates. Still others simply report some “standard error” for all results without regard for the characteristics of the individual analyses, or without providing the basis for the quoted errors. These practices are unacceptable. Zircon data should not be published without realistic and clearly explained error analysis. Uncertainties in 207 Pb ★ 206 Pb ★ ages arise predominantly from two sources: (1) uncertainty in measuring the sample 206 Pb 204 Pb ratio; and (2) uncertainty in the 207 Pb 204 Pb ratio chosen to correct for initial Ph. Errors arising from both of these factors are strongly dependent on the absolute value of the sample 206 Pb 204 Pb ratio, but are otherwise more or less uncorrelated. Several other sources of error are quantifiable, but are of second order importance. Uncertainties in 206 Pb ★ 238 U ages are more complex, in that the quality of the chemical procedures can be a major factor. In fact, the only realistic way of assessing the precision of 206 Pb ★ 238 U ages is via a long-term program of replicate analyses of “well-behaved” natural zircon samples using the exact procedures applied to unknowns. A final commonly overlooked factor in zircon age interpretation, especially interpretation of near-concordant ages, is uncertainty in the accepted uranium decay constants. In most cases, these uncertainties should be treated separately from the random errors of individual analyses. A simple graphical solution is effective.

Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand

The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Ages of Zircons from the Northern Cascade Mountains, Washington

Pre-Late Cretaceous crystalline rocks in the core of the Northern Cascade Mountains of Washington are predominantly heterogeneous supracrustal gneiss and schist, and dioritic, quartz dioritic, and trondhjemitic plutons and bodies of orthogneiss. Migmatite derived from plutonic and supracrustal rocks is also widespread. Isotopic age determinations of zircons indicate that, of the supracrustal gneiss and schist, the Swakane Gneiss and possibly the Skagit Gneiss may have been deposited > 1,650 m.y. ago. Alternatively, some of these rocks may be younger but were derived at least in part from a >l,650-m.y.-old source terrane. A second group of supracrustal rocks, the Younger gneissic rocks of the Holden area, includes a metavolcanic unit about 265 m.y. old. Plutonic rocks and orthogneisses comprise four main age groups: 1,452 to 2,000 m.y., pyroxene gneiss of the Yellow Aster Complex; about 460 m.y., the Turtleback Complex of the San Juan Islands and younger orthogneiss of the Yellow Aster Complex; 220 m.y., plutonic and gneissic rocks of the Marblemount belt (Marblemount Meta-Quartz Diorite, Dumbell Mountain plutons); and 92 m.y., the Eldorado Orthogneiss. Strongly metamorphosed rocks of the Chelan Complex, chiefly quartz dioritic, have zircon Pb 206 /U 238 ages ranging from 100 to 183 m.y. These ages appear to be discordant, and the Chelan Complex may represent remobilized rocks from the 220-m.y.-old Marblemount belt. Ages of metamorphic minerals, zircons from pegmatitic material associated with migmatite, and zircons from synkinematic intrusive rocks indicate two major episodes of metamorphism in the Northern Cascade terrane, one about 415 m.y. ago and the other, 60 to 90 m.y. ago. Both episodes culminated in metamorphism to amphibolite facies grade—and in the case of the older episode, metamorphism to granulite facies grade—and both resulted in extensive migmatization of pre-existing rocks.

Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intra-oceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to sub-aerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the first high-precision U-Pb zircon ages and an extensive new data set of 143Nd/144Nd and 87Sr/86Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ϵNd(t) = 6.0–7.8 and 87Sr/86Srint = 0.703379–0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5–168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ϵNd(t) = 5.6–7.2 and 87Sr/86Srint = 0.703383–0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ϵNd(t) = 4.0–5.5 and 87Sr/86Srint = 0.703656–0.706252] and contain inherited xenocrystic Carboniferous–Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

40Ar/39Ar and U-Pb evidence for late proterozoic (Grenville-age) continental crust in north-central Cuba and regional tectonic implications

Renne, P.R., Mattinson, J.M., Hatten, C.W., Somin, M., Onstott, T.C., Millfin, G. and Linares, E., 1989. 4°Ar/39Ar and U-Pb evidence for Late Proterozoic (Grenville-age) continental crust in north-central Cuba and regional tectonic implications. Precambrian Res., 42: 325-341. Central Cuba is composed of fault-bounded tectonostratigraphic terranes juxtaposed and deformed during plate collision and subsequent transform motion between the Caribbean and North American plates in the Late MesozoicCenozoic. One of these, the Las Villas terrane, contains crystalline basement rocks thought to be pre-Upper Jurassic on stratigraphic grounds. The Socorro Complex occurs in the northwestern Las Villas terrane, and consists of marbles and siliciclastic metasedimentary rocks, and the Rio Carla Granite. An 4°Ar/39Ar plateau date of 903.5 _+ 7.1 Ma for phlogopite from a marble corroborates previous K-Ar dates from this unit, and establishes unambiguously a Late Proterozoic age for high-grade metamorphism. Discordance of the 4°Ar/agAr age spectrum can be reliably attributed to diffusive Ar loss, which if modeled as an episodic thermal event implies a reheating age that closely coincides with the Late-Cretaceous-Paleogene collision between the Caribbean and North American plates. U-Pb zircon data indicate an intrusive age of 172.4 Ma for the Rio Carla Granite, and reveal an inherited zircon component with an age of ~ 900 Ma. Radio-isotopic data from the Socorro Complex display no evidence of Pan-African age thermal overprinting. These observations, combined with constraints provided by published results from nearly Pangean landmasses, suggest that the complex lay substantially to the southwest (present-day co-ordinates) during the Early Paleozoic. Following its genesis in the mid-Mesozoic, the Caribbean plate evidently transported fragments of an extensive Grenville-age belt that spanned the Americas during the Late Proterozoic.