D.A. Archibald

Granite belts in Thailand: evidence from the 40Ar/39Ar geochronological and geological syntheses

Abstract 40 Ar/ 39 Ar radiometric age-dating results together with petrochemical studies of granitoid rocks from nearly all parts of Thailand strongly indicate that the granitoid belts (i.e. Eastern, Central and Western) form in different geological and geotectonic environments and show marked contrast in geochronological evolution. The new 40 Ar/ 39 Ar dating results differ markedly from those previously reported by other methods. The Eastern Granite Belt formed in Early to Late Triassic (245-210 Ma), the Central Belt in Late Triassic (220-180 Ma) to Middle Jurassic, and the Western Belt in Late Cretaceous to Middle Tertiary (80-50 Ma). Granitoid rocks of the S-type which are considered to be related to Sn-W-REE mineralization are widely present in the Western and Central belts as a result of Shan-Thai/Indo-China and Shan-Thai/Western Burma microcontinental plate collisions, respectively. On the other hand, I-type granitoid rocks are inferred to be related to CuFeAuSb mineralization and are largely limited to the Eastern belt, formed as a result of subduction of oceanic lithospheric plates beneath the Shan-Thai and Indo-China microcontinents.

Zongo-San Gabán zone: Eocene foreland boundary of the Central Andean orogen, northwest Bolivia and southeast Peru

Discordant muscovite and biotite K-Ar dates of samples taken in six transects through the Cordillera Oriental of southeast Peru and northwest Bolivia, combined with /sup 40/Ar//sup 39/Ar age spectra, reveal a northwest-trending, structurally cryptic, tectonothermal zone of late Eocene age (ca. 38 Ma) that overprints Triassic and older igneous and metamorphic rocks. The zone is at least 450 km long but is less than 25 km wide; temperatures along its northeast boundary are estimated to have reached 350/sup 0/C. No Paleogene magmatism has been recognized that could account for the thermal resetting of the K-Ar isotopic system. Uplift, erosion, and southwest-verging thrusting accompanied the development of this domain, which defined the foreland boundary of the orogen prior to inception of northeast-verging Andean thrusting in Miocene time. It is proposed that this tectonothermal zone, the Zongo-San Gaban zone (ZSGZ), which involves thrusting and basement shortening, is the southerly manifestation of the Incaic compressional deformation, which produced the thin-skinned Maranon thrust and fold belt (MTFB) north of the Abancay deflection of central Peru.

Delimitation of a cryptic Eocene tectono-thermal domain in the Eastern Cordillera of the Bolivian Andes through K–Ar dating and 40Ar–39Ar step-heating

New K–Ar ages and 40Ar–39 Ar age spectra for samples from granitoid rocks and meta-sediments of the Cordillera Real, Bolivia, reveal a complex mid-Tertiary tectono-thermal history for this segment of the Central Andean eastern cordillera. Previous radiometric dating of the Cordillera Real intrusions defined two, areally-distinct, plutonic domains: in the N, Upper Triassic–Lower Jurassic; in the S, Upper Oligocene–Lower Miocene. Our new K–Ar ages, many from deformed intrusions, fall between these two episodes. For the variably foliated Zongo massif, there is a marked discordance between biotite and muscovite K–Ar dates, each decreasing consistently from SW to NE across this 8 km wide plutonic centre. The age gradients are developed within the foliated and unfoliated facies of the Zongo intrusions, but do not continue NE into the Palaeozoic meta-sedimentary rocks, for which K–Ar dates are erratic. 40Ar–39Ar step-heating analyses of muscovites and most biotites from granitoid rocks reveal that the oldest and youngest micas yield plateaux (210 and 39 Ma), whereas those with intermediate ages exhibit disturbed spectra. A plateau date for a biotite from the NE margin of the Zongo massif shows that final cooling through c. 300 °C occurred in the Late Eocene. The new data reveal that a marked thermal discontinuity existed 39 Ma ago at the NE contact of the Zongo intrusion. Additional discordant dates for the Yani and Illampu intrusions, to the NW, indicate that the discontinuity was of regional scale. There is no direct relationship between this tectono-thermal event, which affected at least 2000 km2, and the Upper Palaeozoic foliation of several of the plutons involved, and thus it is cryptic on outcrop and microscopic scales. We propose that the thermal discontinuity resulted from Late Eocene crustal shortening, SW-verging thrust faulting, and attendant cordilleran uplift at the inner (eastern) boundary of the Andean orogen, coincident with the Incaic deformation defined in Peru.

Late Paleozoic-early Mesozoic magmatism in the Cordillera de Carabaya, Puno, southeastern Peru: Geochronology and petrochemistry

Abstract The Inner Arc domain, the easternmost magmatic manifestation of the post-Paleozoic Central Andean orogeny in southeastern Peru and western Bolivia, comprises a remarkably diverse assemblage of plutonic and volcanic rocks, many of which would be more characteristic of ensialic rifts or collisional mountain belts than of Andean-type convergent plate boundaries. Marked petrologic contrasts with the more homogeneous Main Arc domain, which underlies the westerly provinces of the orogen, have been maintained since the initiation of Andean orogeny in the Late Triassic. Constraints on the chronology and petrogenesis of the early stages in the protracted evolution of the Inner Arc and its Permian antecedents are provided herein by, respectively, KAr and RbSr geochronologic data and major and minor element analyses of representative pre-Cretaceous igneous rocks of the Cordillera de Carabaya, southeastern Peru. Our studies confirm the following sequence of magmatic events, which temporally overlapped with the initial stages of Andean orogeny: 1. i) Intrusion of the gabbroic-to-granitic San Gaban (Corani) complex, a calc-alkaline, but crustally contaminated, suite that cores an extensive area of high-grade, low-pressure metamorphism in lower Paleozoic strata. The complex has been assigned to the mid-Paleozoic, but its age remains poorly defined. The foliated, markedly peraluminous, two-mica granites of the smaller Limacpampa pluton may also have been emplaced during the Paleozoic, but a Triassic age is favored on the basis of our RbSr data. 2. ii) Eruption of alkali basaltic lavas of the Lower Permian Mitu Group along the northeastern margin of a longitudinal ensialic rift that developed in response to extensional tectonism in the interval between the pre-Andean (“late Hercynian”) and Andean orogenies. 3. iii) Emplacement of large granitoid plutons (Coasa, Limbani, and Aricoma centers), with I-Caledonian affinities, along the northeastern boundary of the Mitu rift during the Late Triassic (ca. 225 Ma). The metaluminous to weakly peraluminous monozogranites and granodiorites comprising the greater part of the Carabaya Batholith (new term) were closely associated with mafic dikes of alkaline composition, similar in many respects to the preceding alkali basalts. 4. iv) Development of the Allinccapac Group or peralkaline complex (new term), an assemblage of Middle (and Lower(?) Jurassic lavas, pyroclastics, and plutons exhibiting alkaline to peralkaline affinities. Whereas each of the above suites may be assigned to either a predominant mantle or crustal source, it is evident from the chemical and isotopic data that varied mantle and crustal environments have been involved. Thus, the distinctive chemistries of the coeval granitoid intrusions — as expressed, for example, in the trace element contents of whole-rocks and biotites, the oxidation states of both rocks and biotites, and the initial strontium isotope ratios — demonstrate the contributions of several distinct protoliths. The close spatial and temporal association of mantle and crustal suites during the Permian-to-Jurassic interval strongly implies a cause-and-effect relationship. In particular, the role of basaltic injection in generating large volumes of peraluminous granitoid magmas is amply supported.

Eocene tectono-thermal rejuvenation of an upper Paleozoic-lower Mesozoic terrane in the Cordillera de Carabaya, Puno, southeastern Peru, revealed by KAr and 40Ar/39Ar dating

Abstract KAr dates for muscovites and biotites in granitoid rocks and hydrothermal ore deposits of the northeastern parts of the plutons making up the Triassic Carabaya batholith, underlying the axial Cordillera Oriental of northern Puno Department, southeastern Peru, are markedly variable and mutually discordant. Steep transverse gradients are defined in the apparent ages of both micas, which decrease systematically from SW to NE, delimiting a ca. 25-km-wide, longitudinal zone of anomalously young Mesozoic to Paleocene dates. Age minima of 37±1 Ma are attained in three of the four studied transects. 40 Ar/ 39 Ar step-heating analyses of selected micas confirm the occurrence of a thermal disturbance, and modeling of the spectra suggests that argon loss in muscovites attains at least ca. 75% in the northeastern part of the zone. A single K-feldspar spectrum yielded a minimum at 31 Ma, and apatite fission-track age cluster at ca. 31 and 18.5 Ma. The affected granitoid rocks generally display little megascopic evidence of tectonism, but microscopic deformational fabrics increase in intensity with apparent decreasing KAr age, paralleling a marked increase in alkali feldspar ordering. Secondary fluid inclusions trapped within the microfabrics reveal that the plutonic rocks were penetrated by a homogeneous H 2 OCO 2 CH 4 NaCl fluid at ca. 300–400°C and 0.7–2 kbar. This fluid is implicated in the degassing of the rocks. These diverse data are interpreted as evidence for a major, but moderate-temperature (400°C) and brief, tectono-thermal event at ca. 37±1 Ma (biotite closure temperature)— i.e. , at the Eocene-Oligocene boundary. The K-feldspar 40 Ar/ 39 Ar data and the Oligocene fission-track dates may record the later stages in the event, whereas the Miocene fission-track dates are tentatively ascribed to a distinct Neogene episode. Essentially identical geochronological and petrological relationships have been documented in the Cordillera Real of northwestern Bolivia by McBride et al. (1987), permitting the delimination of a disturbed belt paralleling the South American plate boundary and more than 450 km long. The tectono-thermal domain, which we term the Zongo-San Gaban Zone, constituted the foreland boundary of the Andean orogen in the vicinity of the Arica Deflection during the late Eocene Incaic orogeny. This regional thermal event, which involved the basement, appears to have resulted from compressional or, in some segments, transpressional tectonics.

40Ar/39Ar phlogopite geochronology of lamprophyre dykes in Cornwall, UK: new age constraints on Early Permian post-collisional magmatism in the Rhenohercynian Zone, SW England

The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation of Pangaea. The SW England peninsula is composed largely of Early Devonian to Carboniferous volcano-sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low-grade regional metamorphism during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows, largely on the basis of geochronological data from lamprophyric lavas in Devon. Although published geochronological data for Cornish lamprophyre dykes are consistent with this interpretation, these data are limited largely to imprecise K–Ar whole-rock and biotite analyses, hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional Variscan tectonics. 40Ar/39Ar geochronological data for four previously undated lamprophyre dykes from Cornwall, combined with published data, suggest that lamprophyre magmatism occurred between c. 293.6 and c. 285.4 Ma, supporting previous inferences that their emplacement was coeval with the Cornubian Batholith. These data provide insights into (1) the relative timing between the lamprophyres and basalts, the Cornubian batholith and post-collisional magmatism elsewhere in the European Variscides, and (2) the post-collisional processes responsible for the generation and emplacement of lamprophyres, basalts and granitoids. Supplementary data: Complete datasets, photomicrographs and photographs of sample locations are available online at http://www.geolsoc.org.uk/SUP18838.