Understanding the link between tectonic-driven extensional faulting and volcanism is crucial from a hazard perspective in active volcanic environments, while ancient volcanic successions provide records on how volcanic eruption styles, compositions, magnitudes and frequencies can change in response to extension timing, distribution and intensity. This study draws on intimate relationships of volcanism and extension preserved in the Sierra Madre Occidental (SMO) and Gulf of California (GoC) regions of western Mexico. Here, a major Oligocene rhyolitic ignimbrite “flare-up” (>300,000 km3) switched to a dominantly bimodal and mixed effusive-explosive volcanic phase in the Early Miocene (~100,000 km3), associated with distributed extension and opening of numerous grabens. Rhyolitic dome fields were emplaced along graben edges and at intersections of cross-graben and graben-parallel structures during early stages of graben development. Concomitant with this change in rhyolite eruption style was a change in crustal source as revealed by zircon chronochemistry with rapid rates of rhyolite magma generation due to remelting of mid- to upper crustal, highly differentiated igneous rocks emplaced during earlier SMO magmatism. Extension became more focused ~18 Ma resulting in volcanic activity being localised along the site of GoC opening. This localised volcanism (known as the Comondu “arc”) was dominantly effusive and andesite-dacite in composition. This compositional change resulted from increased mixing of basaltic and rhyolitic magmas rather than fluid flux melting of the mantle wedge above the subducting Guadalupe Plate. A poor understanding of space-time relationships of volcanism and extension has thus led to incorrect past tectonic interpretations of Comondu-age volcanism.
The Devonian subsurface Adavale Basin occupies a central position in the Paleozoic central Thomson Orogen of eastern Australia and records its tectonic setting during this time interval. Here, we have focussed on the basal volcanics of the Gumbardo Formation to clarify the tectonic setting of the basin. The approach has been to undertake stratigraphic logging, LA-ICP-MS U–Pb zircon geochronology and whole-rock geochemical analysis. The data indicate that basin initiation was rapid occurring at ca 401 Ma. The volcanic rocks are dominated by K-feldspar phyric rhyodacitic ignimbrites. The whole-rock geochemical data indicate little evidence for extensive fractional crystallisation, with the volcanic suite resembling the composition of the upper continental crust and exhibiting transitional I- to A-type tectonomagmatic affinities. One new U–Pb zircon age revealed an Early Ordovician emplacement age for a volcanic rock previously interpreted to be part of the Early Devonian Gumbardo Formation, and older basement age is consistent with seismic interpretations of uplifted basement in this region of the western Adavale Basin. Five ignimbrites dated from different stratigraphic levels within the formation yield similar emplacement ages with a pooled weighted age of 398.2 ± 1.9 Ma (mean square weighted deviation = 0.94, n = 93). Significant zircon inheritance in the volcanic rocks records reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen and provides insight into the crustal make-up of the Thomson Orogen. Collectively, the new data presented here suggest the Adavale Basin is a cover-type basin that developed on a stabilised Thomson Orogen after the major Bindian deformation event in the late Silurian.
Isotopic analyses were undertaken to clarify age relationships between mineralization in the Pueblo Viejo high-sulfidation epithermal deposit and its host Los Ranchos Formation. This study is important to models for high-sulfidation epithermal deposits because the Los Ranchos Formation is a largely submarine, island-arc tholeiite sequence, very unlike the subaerial, calc-alkaline volcanic sequences that host most high-sulfidation epithermal deposits. Previous U-Pb analyses of zircons from the Los Ranchos Formation show that it formed between about 118 and 111 Ma and was intruded by the Cotui stock at about 112 Ma. New U-Pb isotope analyses of zircons from quartz porphyries at two different levels in the ore-hosting, upper Los Ranchos Formation indicate an age of 111.4 ± 0.5 Ma (MSWD = 1.92), essentially the same as that of the Cotui stock, confirming that the stock was probably the source of the quartz porphyry magmas and mineralizing fluids. Rb-Sr analyses of sphalerite from gold-bearing veins reflect mixing between magmatic and seawater Sr reservoirs but yield no age constraints. Ar-Ar analyses of illite from advanced argillic alteration in the Moore orebody yield a plateau age of 58 Ma interpreted to represent complete resetting or neocrystallization, probably related to the thermal effect of small diorite intrusions immediately south and west of the district. These measurements, combined with geologic relationships, confirm that high-sulfidation mineralization at Pueblo Viejo was coeval with deposition of its host Los Ranchos Formation, part of the island-arc tholeiite sequence that forms the base of the Greater Antilles arc. This greatly expands the types of volcanic terranes that might be prospective for high-sulfidation mineralization beyond subaerial calc-alkaline sequences to more mafic, dominantly submarine, island-arc tholeiite sequences. At present, relatively few high-sulfidation deposits are known in island-arc tholeiite sequences. This could reflect the greater difficulty of developing large advanced argillic alteration zones in these dominantly mafic rocks or simply a failure to recognize island-arc tholeiite sequences in volcanic arcs that contain mineralization. Exploration might also have been discouraged by the fact that most island-arc tholeiite sequences are dominantly submarine and contain exhalative mineralization. That both types of mineralization might form in these arcs is suggested by relationships in the Tonga-Kermadec ridge and adjacent areas, where dacitic calderas in island-arc tholeiite sequences host exhalative and high-sulfidation mineralization. The presence of an unusually large deposit such as Pueblo Viejo in an island-arc volcanic sequence is attributable to the iron-rich nature of the wall rocks, which promoted early deposition of gold by sulfidation, and to the presence of carbonaceous sedimentary rocks that isolated the hydrothermal system and promoted ore deposition. The age and isotopic data show further that these carbonaceous sedimentary rocks were deposited during ocean anoxic event (OAE) 1b, encouraging speculation that other island-arc tholeiite volcanic sequences containing ocean anoxic event sediments might also be favorable for high-sulfidation epithermal mineralization.
A growing body of evidence indicates that Middle Jurassic to Early Cretaceous plutons recorded changing sources during tectonic evolution of the Klamath Mountain province. The data set now includes U-Pb zircon ages and zircon trace element compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Thirteen rock samples were dated, and these data refine thermal ionization mass spectrometry (TIMS) data where inheritance was problematic, or provide new U-Pb ages. Individual plutonic suites, previously defined on the basis of crystallization age, isotope and elemental compositions, and petrogenetic style, show characteristic inherited zircon age ranges and zircon trace element patterns. Moreover, ages of inherited zircons in these suites are distinct and, in at least three suites, indicate the presence of cryptic (unexposed) source rocks. The zircon data complement oxygen, Nd, and Sr isotope whole-rock data that, when taken together, suggest a number of major changes in the crustal column with time. Middle Jurassic magmatism began with the oceanic(?) arc-related western Hayfork terrane comprising volcanic, volcaniclastic, and plutonic components. After regional thrusting on the ca. 170-Ma Wilson Point thrust, the Ironside Mountain batholith and Wooley Creek suite of plutons were emplaced. The former shows little evidence of interaction with the crust, but the latter contains Middle Jurassic inheritance and Sr, Nd, and oxygen isotope signatures suggestive of interaction with metasedimentary crustal rocks. Following Nevadan thrusting (ca. 153–150 Ma), emplacement of western Klamath suite plutons in the western Klamath Mountains province involved significant assimilation of Galice Formation metasedimentary rocks. This activity was followed by emplacement of tonalite-trondhjemite-granodiorite (ttg) plutons in the eastern Klamath Mountains province, which were derived by partial melting of metabasic rocks. Their zircon trace element signatures indicate diverse magma histories and, at least locally, multiple magma sources. Inherited zircons in ttg plutons suggest late Middle Jurassic to Late Jurassic sources, younger than the Josephine ophiolite. The youngest magmatism in the Klamath Mountains province consists of broadly granodioritic plutons, which, on the basis of limited data, show variable petrogenesis and zircon inheritance. At least one of these plutons (136-Ma Yellow Butte pluton) contains a ca. 150-Ma inheritance that indicates the presence of Late Jurassic crustal rocks beneath the eastern Klamath terrane.
U‐Pb and K‐Ar age determinations on igneous rocks have been used to trace the Late Palaeozoic to Mesozoic magmatic and tectonic history of the northern New England Orogen, northeastern Australia. The oldest igneous rocks in the Connors Arch, tonalites of the Urannah Suite, yield zircon ages of 304.3 ± 5.8 and 308.2 ± 7.1 Ma. One of a swarm of felsic dykes chemically related to the Urannah Suite granites is distinctly younger at 283.9 ± 5.2 Ma. The cross‐cutting Thunderbolt Granite is slightly younger again at 277.9 ± 6.3 Ma. The granites of the Urannah Suite are not the remnants of a Late Devonian ‐ mid‐Carboniferous arc. A zircon age of 294.2 ± 2.8 Ma measured on an ignimbrite from the Carmila beds at Dumbleton Rocks indicates initiation of the adjacent Bowen Basin during, or soon after, the main Late Carboniferous batholith‐forming event. A later generation of more isotopically primitive magmatism is represented by younger plutons and mafic dykes. The Triassic Gloucester Granite east of the Connors Arch has a zircon age of 243.5 ± 4.9 Ma. In the arch itself, the Cretaceous Hecate and Mt Abbot Granites have been dated at 130.8 ± 3.4 and 119.3 ± 2.2 Ma, respectively. The Blackwall Quartz Diorite in the Bowen Basin to the west is of similar age, 132.5 ± 2.4 Ma. K‐Ar mineral ages measured on six chemically distinctive 'Cretaceous' granites in the Connors Arch range from 145.3 to 103.3 Ma, substantially extending the known time span of 'Cretaceous' magmatism in the region. The entire Late Carboniferous to Triassic magmatic episode occurred during a protracted period of crustal extension that appears to have been terminated by a major compressional event, the Hunter‐Bowen Orogeny. The initiation of Cretaceous magmatism significantly pre‐dates the opening of the Tasman Sea.
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