Timing Of Plutonism Research Articles

Timing of plutonism in the Gällivare area: implications for Proterozoic crustal development in the northern Norrbotten ore district, Sweden

AbstractZircon ion probe (secondary-ion mass spectrometry or SIMS) data from a set of intrusive rocks emplaced in the vicinity of major ore bodies, as well as from large igneous intrusions in the Gällivare area, gave the following results: (1) the Dundret ultramafic–mafic layered complex (1883±5 Ma), the Aitik granite (1883±5 Ma), the Nautanen diorite (1870±12 Ma), the Vassaravaara ultramafic–mafic layered complex (1798±4 Ma), the Aitik dolerite (1813±9 Ma), the Bergmästergruvan and Sikträsk syenites (1795±4 Ma and 1801±3 Ma, respectively) and the Naalojärvi granite (1782±5 Ma). These data broadly fall within the ranges 1.89–1.87 Ga (early Svecofennian) and 1.80–1.78 Ga (late Svecofennian), but geochronologically allow further subdivision into pulses at 1885–1880, 1875–1870, 1800 and 1780 Ma. During these events, large layered ultramafic–mafic and felsic plutonic rocks were generated with distinct overlap in time suggesting coeval felsic–mafic magmatism. Results also indicate the presence of inheritedc.1.87 Ga zircon crystals in the plutonic rocks at 1.78 Ga, supporting reworking of the previous crust. These data indicate the importance of mantle-derived mafic underplating in the process of crustal magma generation in the region. Thec.1.88 Ga event that generated ultramafic–mafic layered complexes is tentatively suggested to have played an important role in the formation of the Aitik Cu–Au porphyry system. The later event atc.1.80 Ga, generating voluminous mafic–felsic units, is suggested to be coupled to the regional iron-oxide-copper-gold (IOCG) overprint.

Timing of plutonism and deformation in the White Mountains of eastern California

New mapping and U-Pb zircon geochronology help establish the timing of contractional deformation and magmatism in the White Mountains of California. In the Redding Canyon area of the west-central White Mountains, Mesozoic deformation characterized by east-directed movement along reverse faults as well as recumbent folding was followed by development of upright folds with axes that plunge moderately to the north. This later folding event produced penetrative, vertical, north-striking axial-planar cleavage that is present along much of the western flank of the range. Deformed units include folded and/or boudinaged diorite dikes (ca. 165 Ma; U-Pb zircon) that contain the later penetrative cleavage. The cleavage is clearly cut by the Redding Canyon pluton (ca. 164 Ma; U-Pb zircon), demonstrating that at least some of the intense deformation preserved in the area is Middle Jurassic and correlative with the East Sierran thrust system identified elsewhere in California. Dates for the Beer Creek pluton (ca. 179 Ma; U-Pb zircon) and the Sage Hen Flat pluton (ca. 175 Ma; U-Pb zircon), which cut deformation in their wall rocks, suggest that East Sierran thrust deformation did not propagate as far eastward as these plutons at the present level of exposure. The new dates also cast doubt on the presence of any Late Jurassic–Early Cretaceous plutonism in the White Mountains. Throughout the east-central Sierra Nevada and White Mountains, high-precision U-Pb zircon geochronology is resolving significant plutonism into two short-lived events that occurred at ca. 180–165 Ma and 102–86 Ma.

U–Pb geochronological constraints on the timing of plutonism, volcanism, and sedimentation, Jersey, Channel Islands, UK

Uranium–lead dates for single and small multiple grain zircon separates are presented for the main geological units from Jersey, British Channel Islands. A turbiditic sandstone from the Jersey Shale Formation (Brioverian Supergroup) contains detrital zircons of a variety of ages, ranging from c. 2.6 to 0.6 Ga. The youngest zircons yielded nearly concordant U–Pb dates of 586.7±3.0 Ma and 587.1±2.7 Ma providing a maximum depositional age for the unit. Zircons from overlying volcanic rocks (Anne Port Rhyolite) give an upper intercept date of 582.8 +3 −2.7 Ma, considered to be the age of eruption. The undeformed SW Jersey granite, which intrudes and locally thermally metamorphoses the folded Jersey Shale Formation, is dated at 580 +2.3 −1.6 Ma. These dates constrain tightly the timing of deposition of the Jersey Shale Formation and the age of deformation on Jersey, and indicate a dynamic arc environment during the late Neoproterozoic. Zircons from the NW Jersey granite define an upper intercept date of 482.7±1.6 Ma, similar to the previously reported 40 Ar/ 39 Ar hornblende date of c. 473 Ma, confirming a renewed pulse of magmatism during the Ordovician in this sector of Cadomia.

Timing of plutonism, deformation, and metamorphism in the Yellowknife Domain, Slave Province, Canada

U-Pb zircon, titanite, and monazite ages for plutonic rocks are used to constrain the timing of post-2660 Ma deformational and metamorphic events in the Yellowknife Domain, southern Slave Province. Diorite and tonalite plutons of the Defeat Suite yield ages of 2628 ± 3 and 2624 ± 3 Ma, older than previous, less precise estimates for this suite of ca. 2618-2620 Ma. The Defeat Suite plutons crosscut and postdate upright F1 folds in turbidites of the ca. 2680-2660 Ma Burwash Formation. D1 deformation is therefore bracketed between 2660 and 2630 Ma. The D1 event significantly predates deposition of polymict conglomerates of the Jackson Lake Formation. A second folding event (F2), represented by northwest- to north-trending steeply plunging folds of bedding and D1 structures, affected the area after intrusion of the Hidden Lake pluton at 2606 ± 2 Ma, and synchronously with intrusion of the 2596 ± 2 Ma Sparrow Lake pluton, a member of the two-mica granite Prosperous suite. The time of initiation and the duration of D2 deformation are not well defined. It may have affected units as young as the 2586 ± 2 Ma Morose Granite. Metamorphic isograds are spatially related to Defeat, Prosperous, or even younger plutonic complexes, and therefore formed at different times over a 30 Ma period in different parts of the Yellowknife Domain. The ca. 2620-2630 Ma Defeat-type plutonism started in the Yellowknife Domain 10-15 Ma earlier than similar intrusive events in the central and northern Slave Province and highlights diachronous development of plutonism in the period from 2630 to 2610 Ma. In contrast, the ages of younger granites in the Yellowknife Domain (Prosperous, Morose) are within the range of intrusive ages documented for similar plutonic suites elsewhere in the Slave Province.

Timing of plutonism, deformation, and metamorphism in the Yellowknife Domain, Slave Province, Canada

U-Pb zircon, titanite, and monazite ages for plutonic rocks are used to constrain the timing of post-2660 Ma deformational and metamorphic events in the Yellowknife Domain, southern Slave Province. Diorite and tonalite plutons of the Defeat Suite yield ages of 2628 ± 3 and 2624 ± 3 Ma, older than previous, less precise estimates for this suite of ca. 2618-2620 Ma. The Defeat Suite plutons crosscut and postdate upright F1 folds in turbidites of the ca. 2680-2660 Ma Burwash Formation. D1 deformation is therefore bracketed between 2660 and 2630 Ma. The D1 event significantly predates deposition of polymict conglomerates of the Jackson Lake Formation. A second folding event (F2), represented by northwest- to north-trending steeply plunging folds of bedding and D1 structures, affected the area after intrusion of the Hidden Lake pluton at 2606 ± 2 Ma, and synchronously with intrusion of the 2596 ± 2 Ma Sparrow Lake pluton, a member of the two-mica granite Prosperous suite. The time of initiation and the duratio...

Timing of plutonism in the Proterozoic Albany Mobile Belt, southwestern Australia

Abstract Zircon U-Pb ages for two adamellites from the Albany Mobile Belt, in the Albany-Fraser Orogen of southwestern Australia, fall within the range 1174 ± 12 Ma and are interpreted as dating post-tectonic granite emplacement in the belt. Slightly younger K-Ar ages on hornblendes from granulite and amphibolite facies metamorphic rocks from the southern part of the belt are interpreted as cooling ages recording uplift associated with the closing stages of orogeny and granite emplacement. A U-Pb age of 1289 ± 10 Ma for zircons from an enderbitic gneiss adjacent to the 1177 ± 12 Ma old Albany Adamellite is interpreted as dating the emplacement of the granodioritic parent of the gneiss, probably during granulite facies metamorphism. This, together with a previously reported Rb-Sr age of ∼ 1300 Ma for the granulite facies metamorphism in the northeast section of the Albany-Fraser Orogen, suggests that granulite facies conditions were widespread in the orogen at ∼ 1300 Ma. Previous Sm-Nd results suggest that the granitoids were derived from ∼ 2000 Ma old crust. So far no older zircon xenocrysts, which could support this model, have been found in the granitoids.

Janet Watson—an appreciation and bibliography

Janet Watson—an appreciation and bibliography

A contemplation of time in plutonism

Summary A general survey is first made of certain aspects of time in plutonism. These include the duration of plutonic times, their limits and subdivisions, the different dates concerned in them, and the unity or otherwise of plutonic processes. More specific inquiries begin with the consideration of time and crystallization. The criteria for the determination of time-sequences in the crystallization of the plutonic rocks are reviewed and examples of their application given. The more complex question of the time-relations of crystallization and deformation is next taken up, chief attention being given to the evidence for the rotation of porphyroblasts. The diagnostic requirements for pre-crystalline, para-crystalline and post-crystalline deformations are examined. The inquiry next moves to time-relationships of a different order, those relating plutonism and orogeny. The implications of folding of a metamorphic succession and the validity of inversion in the crystalline schists are discussed. The importance of metamorphic history, facies and convergence in geological interpretation is illustrated by examples from Scotland, central Europe and elsewhere. The problem of the subdivision of the metamorphic history of a rock and the evidence for monometamorphism or polymetamorphism are considered and examples from the Scottish Highlands are discussed. As the whole operation of plutonism depends upon the activity of migma-magma, it is necessary to deal with the time-relations of crystallization, deformation, granitization, migmatization, metamorphism, intrusion and orogeny. The general principles of the relation between magmatism and orogeny are explored. The micro tectonic and deep-tectonic studies of Demay are summarized and discussed— the discussion leading on to the consideration of the characters of synkinematic intrusion and of synkinematic permeation. Finally, the Granite Series is displayed and discussed. This attempts to relate the plutonic phenomena at the various levels of exposure, and to give a unity to the processes of granitization, migmatization and metamorphism at depth and at successively higher positions and later times. The series begins with the autochthonous granites associated with niigmatites and metamorphites produced in situ in the depths. The movement of the granitized material provides the successive terms of the series. The parting of envelope and core, the structural control of movement, straining off, selective mobilization, overtaking and chemical variation in the series are discussed. The Granite Series ends with the late high-level plutons emplaced as viscous masses. An example of the Granite Series from the Hercynian belt of north-western Europe is presented. The plutonic developments in the Massif Central, Armorica and South-West England are compared. In this belt, the,later members of the Granite Series appear at higher levels as we pass from south to north, and there is a progressive change in the style of contact in the same direction. The higher and younger members to the north have left their deeper and earlier roots in the Massif Central. At the three or four sample levels of the Granite Series provided by this example, fundamental differences of pattern reveal the relationships between place and time in the making of the patterns.