Christopher L. Andronicos

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Large‐scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia

The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ∼110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ∼101 and ∼85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ∼2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30′N.

Near-isothermal conditions in the middle and lower crust induced by melt migration

The thermal structure of the crust strongly influences deformation, metamorphism and plutonism. Models for the geothermal gradient in stable crust predict a steady increase of temperature with depth. This thermal structure, however, is incompatible with observations from high-temperature metamorphic terranes exhumed in orogens. Global compilations of peak conditions in high-temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure, for both isothermal decompression and isobaric cooling paths. Here we develop simple one-dimensional thermal models that include the effects of melt migration. These models show that long-lived plutonism results in a quasi-steady-state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust. The models also predict that the upward advection of heat by melt generates granulite facies metamorphism, and widespread andalusite–sillimanite metamorphism in the upper crust. Once the quasi-steady-state thermal profile is reached, the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions, thus setting the stage for gravitational collapse, exhumation and isothermal decompression after the onset of plutonism. Near-isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that, in part, control the shape of the geotherm.

Natural quasicrystal with decagonal symmetry

We report the first occurrence of a natural quasicrystal with decagonal symmetry. The quasicrystal, with composition Al71Ni24Fe5, was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite. Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal to be identified, was found in the same meteorite. The new quasicrystal was found associated with steinhardtite (Al38Ni32Fe30), Fe-poor steinhardtite (Al50Ni40Fe10), Al-bearing trevorite (NiFe2O4) and Al-bearing taenite (FeNi). Laboratory studies of decagonal Al71Ni24Fe5 have shown that it is stable over a narrow range of temperatures, 1120 K to 1200 K at standard pressure, providing support for our earlier conclusion that the Khatyrka meteorite reached heterogeneous high temperatures [1100 < T(K) ≤ 1500] and then rapidly cooled after being heated during an impact-induced shock that occurred in outer space 4.5 Gya. The occurrences of metallic Al alloyed with Cu, Ni, and Fe raises new questions regarding conditions that can be achieved in the early solar nebula.

Impact-induced shock and the formation of natural quasicrystals in the early solar system

The discovery of a natural quasicrystal, icosahedrite (Al63Cu24Fe13), accompanied by khatyrkite (CuAl2) and cupalite (CuAl) in the CV3 carbonaceous chondrite Khatyrka has posed a mystery as to what extraterrestrial processes led to the formation and preservation of these metal alloys. Here we present a range of evidence, including the discovery of high-pressure phases never observed before in a CV3 chondrite, indicating that an impact shock generated a heterogeneous distribution of pressures and temperatures in which some portions reached at least 5 GPa and 1,200 °C. The conditions were sufficient to melt Al-Cu-bearing minerals, which then rapidly solidified into icosahedrite and other Al-Cu metal phases. The meteorite also contains heretofore unobserved phases of iron-nickel and iron sulphide with substantial amounts of Al and Cu. The presence of these phases in Khatyrka provides further proof that the Al-Cu alloys are natural products of unusual processes that occurred in the early solar system.

Shear velocity structure beneath the central United States: Implications for the origin of the Illinois Basin and intraplate seismicity

We present new estimates of lithospheric shear velocities for the intraplate seismic zones and the Illinois Basin in the US midcontinent by analyzing teleseismic Rayleigh waves. We find that relatively high crustal shear velocities (VS) characterize the southern Illinois Basin, while relatively low crustal velocities characterize the middle and lower crust of the central and northern Illinois Basin. The observed high crustal velocities may correspond to high-density mafic intrusions emplaced into the crust during the development of the Reelfoot Rift, which may have contributed to the subsidence of the Illinois Basin. The low crustal VS beneath the central and northern basin follow the La Salle deformation belt. We also observe relatively low velocities in the mantle beneath the New Madrid seismic zone where VS decreases by about 7% compared to those outside of the rift. The low VS in the upper mantle also extends beneath the Wabash Valley and Ste. Genevieve seismic zones. Testing expected VS reductions based on plausible thermal heterogeneities for the midcontinent indicates that the 7% velocity reduction would not result from elevated temperatures alone. Instead this scale of anomaly requires a contribution from some combination of increased iron and water content. Both rifting and interaction with a mantle plume could introduce these compositional heterogeneities. Similar orientations for the NE-SW low-velocity zone and the Reelfoot Rift suggest a rift origin to the reduced velocities. The low VS upper mantle represents a weak region and the intraplate seismic zones would correspond to concentrated crustal deformation above weak mantle. This article is protected by copyright. All rights reserved.

The Central Gneiss Complex, Coast Mountains, British Columbia

The Central Gneiss Complex of British Columbia, between lat 54° and 55°N, consists mainly of orthogneiss, rusty-weathering migmatite, leucogneiss, amphibolite, and minor calc-silicate. It forms the country rock for the large intrusions of tonalite and granodiorite that constitute the Coast Mountains batholith. With the exception of calc-silicates, all lithologies intruded by the Paleogene plutons have been partially melted to varying degrees. Metamorphic grade within the complex varies from upper amphibolite facies to granulite facies. The granulite facies rocks are in close proximity to the margins of the large tonalite-granodiorite plutons, suggesting that the plutons locally heated gneisses to the granulite facies. The geologic structure of the Central Gneiss Complex is the result of sequential regional deformations related to the emplacement of voluminous tonalite-granodiorite plutons. Much of the structure can be interpreted to be due to progressive deformation during dextral transpression, with a final imprint produced by flattening during extension. Kilometer-scale tight to isoclinal folds with steeply east-dipping axial planes and shallow to moderately north-plunging hinges were formed during east-side-up shearing with a dextral component along the eastern side of the complex. Transfer of strain from the eastern side of the complex to the western side was accommodated by top-to-the-west shearing that produced reclined isoclinal folds with east-plunging fold axes and shallowly north-dipping axial planes that constitute a flat between two ramps. A domain of steeply dipping foliation dominates the western side of the complex and is cored by the 58.6 Ma Quottoon pluton. Dextral strike-slip shearing occurred in the country rock along the eastern side of Quottoon pluton, and east-side up shearing occurred within the Quottoon pluton. The Quottoon pluton intruded late during transpressive deformation. Intrusion of the Kasiks sill (53 Ma) resulted in flattening of the underlying gneisses, and the interference pattern of this late flattening with foliations produced by the earlier transpressive deformation produced a foliation triple point to the south of the sill. The occurrence of a distinctive lithologic package crossing the Central Gneiss Complex restricts any major zones of strike-slip displacement to be located along its eastern side or along its western side; dextral strike slip strains are recorded along the western side. These features suggest that if the Baja British Columbia fault system passes through the Central Gneiss Complex, it is likely to be where the Quottoon pluton is currently located.

Steinhardtite, a new body-centered-cubic allotropic form of aluminum from the Khatyrka CV3 carbonaceous chondrite

Abstract Steinhardtite is a new mineral from the Khatyrka meteorite; it is a new allotropic form of aluminum. It occurs as rare crystals up to ~10 μm across in meteoritic fragments that contain evidence of a heterogeneous distribution of pressures and temperatures during impact shock, in which some portions of the meteorite reached at least 5 GPa and 1200 °C. The meteorite fragments contain the high-pressure phases ahrensite, coesite, stishovite, and an unnamed spinelloid with composition Fe3-xSixO4 (x ≈ 0.4). Other minerals include trevorite, Ni-Al-Mg-Fe spinels, magnetite, diopside, forsterite, clinoenstatite, nepheline, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, cupalite, taenite, and Al-bearing taenite. Given the exceedingly small grain size of steinhardtite, it was not possible to determine most of the physical properties for the mineral. A mean of 9 electron microprobe analyses (obtained from two different fragments) gave the formula Al0.38Ni0.32Fe0.30, on the basis of 1 atom. A combined TEM and single-crystal X‑ray diffraction study revealed steinhardtite to be cubic, space group Im3m, with a = 3.0214(8) Å, and V = 27.58(2) Å3, Z = 2. In the crystal structure [R1 = 0.0254], the three elements are disordered at the origin of the unit cell in a body-centered-cubic packing (α-Fe structure type). The five strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are: 2.1355 (100) (110); 1.5100 (15) (200); 1.2329 (25) (211); 0.9550 (10) (310); 0.8071 (30) (321). The new mineral has been approved by the IMA-NMNC Commission (2014-036) and named in honor of Paul J. Steinhardt, Professor at the Department of Physics of Princeton University, for his extraordinary and enthusiastic dedication to the study of the mineralogy of the Khatyrka meteorite, a unique CV3 carbonaceous chondrite containing the first natural quasicrystalline phase icosahedrite. The recovery of the polymorph of Al described here that contains essential amounts of Ni and Fe suggests that Al could be a contributing candidate for the anomalously low density of the Earth’s presumed Fe-Ni core.

Syntectonic 1.46 Ga magmatism and rapid cooling of a gneiss dome in the southern Mazatzal Province: Burro Mountains, New Mexico

Large-volume granitic plutons in the Burro Mountains, southwestern New Mexico, cover an area of 1300 km 2 and include biotite leucogranite and biotite-hornblende granodiorite. These intrusions are part of the ca. 1.4 Ga granite and rhyolite province stretching across Laurentia. U-Pb zircon dating of five samples of the biotite leucogranite yielded ages ranging from 1469 ± 12 to 1455 ± 11 Ma (2σ uncertainty). Three samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the zircon ages are within error and together have a weighted mean age of 1462 ± 3 Ma, but the granodiorite is older than the granite based on crosscutting relationships. Pressure and temperature estimates from metapelitic country rock, the first from southern New Mexico, are 4–6.5 kbar and 575–650 °C, consistent with their mineralogy. Decompression textures on garnets indicate exhumation following peak conditions, interpreted to have been reached during magmatic heating at 1460 Ma. Decompression had a magnitude of 1–3 kbar. An amphibolite from the country rock has metamorphic zircons dated at 1458 ± 9 Ma, with low Th/U and soccer-ball morphology enclosed within hornblende; this is the age of amphibolite-facies metamorphism. Electron microprobe monazite U-Pb dates from metapelite country rock mostly range from 1500 to 1400 Ma, with a peak at 1474 Ma. This is interpreted as indicating that the area experienced high-temperature metamorphism associated with pluton emplacement at this time. The 40 Ar/ 39 Ar dates from granite and amphibolite form two main groups. The older group includes four hornblende dates of 1477–1467 Ma and biotite and white mica ages from 1472 to 1459 Ma. These are interpreted as reflecting cooling of the plutons and their country rock soon after intrusion. The younger group of hornblende and biotite ages is at 1228 ± 3 Ma and is close in age to the adjacent Redrock granite, which intruded at 1225 Ma. There is a dominant strong foliation in the country rock gneisses and metasedimentary rocks. Foliated granite and gabbro both yield ca. 1630 Ma U-Pb zircon ages. The Burro Mountain granite is not pervasively deformed, but the granodiorite has strong augen gneissic foliations and mylonitic shear zones with S-C fabrics. This fabric formed prior to intrusion of undeformed dikes of Burro Mountain granite and thus is synmagmatic within geochronologic uncertainty. The fabric is identical to the fabric in the country rock, and xenoliths of country rock are foliated with orientations parallel to the foliation in the granodiorite. The similar orientations and the timing of metamorphism suggest that most of the deformation in the Burro Mountains occurred at 1460 Ma and was synchronous with intrusion of the granodiorite. We interpret the 1460 Ma plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development within an extensional tectonic setting, although a transpressional setting is also permissible.

Decagonite, Al71Ni24Fe5, a quasicrystal with decagonal symmetry from the Khatyrka CV3 carbonaceous chondrite

Abstract Decagonite is the second natural quasicrystal, after icosahedrite (Al63Cu24Fe13), and the first to exhibit the crystallographically forbidden decagonal symmetry. It was found as rare fragments up to ~60 mm across in one of the grains (labeled number 126) of the Khatyrka meteorite, a CV3 carbonaceous chondrite. The meteoritic grain contains evidence of a heterogeneous distribution of pressures and temperatures that occurred during impact shock, in which some portions of the meteorite reached at least 5 GPa and 1200 °C. Decagonite is associated with Al-bearing trevorite, diopside, forsterite, ahrensite, clinoenstatite, nepheline, coesite, pentlandite, Cu-bearing troilite, icosahedrite, khatyrkite, taenite, Al-bearing taenite, and steinhardtite. Given the exceedingly small size of decagonite, it was not possible to determine most of the physical properties for the mineral. A mean of seven electron microprobe analyses (obtained from three different fragments) gave the formula Al70.2(3)Ni24.5(4)Fe5.3(2), on the basis of 100 atoms. A combined TEM and single-crystal X‑ray diffraction study revealed the unmistakable signature of a decagonal quasicrystal: a pattern of sharp peaks arranged in straight lines with 10-fold symmetry together with periodic patterns taken perpendicular to the 10-fold direction. For quasicrystals, by definition, the structure is not reducible to a single three-dimensional unit cell, so neither cell parameters nor Z can be given. The likely space group is P105/mmc, as is the case for synthetic Al71Ni24Fe5. The five strongest powder-diffraction lines [d in Å (I/I0)] are: 2.024 (100), 3.765 (50), 2.051 (45), 3.405 (40), 1.9799 (40). The new mineral has been approved by the IMA-NMNC Commission (IMA2015-017) and named decagonite for the 10-fold symmetry of its structure. The finding of a second natural quasicrystal informs the longstanding debate about the stability and robustness of quasicrystals among condensed matter physicists and demonstrates that mineralogy can continue to surprise us and have a strong impact on other disciplines.