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1.
墨西哥西马德雷山脉是白垩纪—新生代岩浆活动和构造运动形成的。中新生代岩浆活动可以分为5个主要阶段:侏罗纪—早白垩世、晚白垩世—古新世、始新世—渐新世、中新世早期和中新世中期—现代。这些岩浆活动和构造运动与法拉隆(Farallon)板块向北美大陆俯冲和加利福尼亚湾打开相关。墨西哥中新生代的成矿作用与东太平洋板块边缘连续的俯冲过程密切相关,矿床类型多样,包括VMS(与火山相关的块状硫化物)型、斑岩型、IOCG(铁氧化物铜金)型、矽卡岩型等。  相似文献   

2.
The Sierra Madre Occidental of northwestern Mexico is the biggest silicic large igneous province of the Cenozoic, yet very little is known about its geology due to difficulties of access to much of this region. This study presents geologic maps and two new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages from the Cerocahui basin, a previously unmapped and undated ~25 km-long by ~12 km-wide half-graben along the western edge of the relatively unextended core of the northern Sierra Madre Occidental silicic large igneous province. Five stratigraphic units are defined in the study area: (1) undated welded to non-welded silicic ignimbrites that underlie the rocks of the Cerocahui basin, likely correlative to Oligocene-age ignimbrites to the east and west; (2) the ca. 27.5–26 Ma Bahuichivo volcanics, comprising mafic-intermediate lavas and subvolcanic intrusions in the Cerocahui basin; (3) alluvial fan deposits and interbedded distal non-welded silicic ignimbrites of the Cerocahui clastic unit; (4) basalt lavas erupted into the Cerocahui basin following alluvial deposition; and (5) silicic hypabyssal intrusions emplaced along the eastern margin of the basin and to a lesser degree within the basin deposits.

The main geologic structures in the Cerocahui basin and surrounding region are NNW-trending normal faults, with the basin bounded on the east by the syndepositional W-dipping Bahuichivo–Bachamichi and Pañales faults. Evidence of syndepositional extension in the half-graben (e.g. fanning dips, unconformities, coarsening of clastic deposits toward basin-bounding faults) indicates that normal faulting was active during deposition in the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas), and may have been active earlier based on regional correlations.

The rocks in the Cerocahui basin and adjacent areas record: (1) the eruption of silicic outflow ignimbrite sheets, likely erupted from caldera sources to the east during the early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up, mostly prior to synextensional deposition in the Cerocahui basin (pre-27.5 Ma); (2) synextensional late Oligocene mafic-intermediate composition magmatism and alluvial fan sedimentation (ca. 27.5–24.5 Ma), which occurred during the lull between the Early Oligocene and early Miocene pulses of the ignimbrite flare-up; and (3) post-extensional emplacement of silicic hypabyssal intrusions along pre-existing normal faults, likely during the early Miocene pulse of the ignimbrite flare-up (younger than ca. 24.5 Ma). The timing of extensional faulting and magmatism in the Cerocahui basin and surrounding area generally coincides with previous models of regional-scale middle Eocene to early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental controlled by post-late Eocene (ca. 40 Ma) rollback/fallback of the subducted Farallon slab.  相似文献   

3.
The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and 40Ar/39Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. 40Ar/39Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (40Ar/39Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.  相似文献   

4.
Geologic mapping and new K-Ar and 40Ar/39Ar geochronology of the southeastern Sierra Madre Occidental (SMO), at its intersection with the northern margin of the Mexican Volcanic Belt (MVB), indicate the occurrence of three volcanic groups. The oldest group corresponds to the SMO, and includes 29 to 22 Ma voluminous ignimbrites and 30 Ma andesites and rhyolites. The youngest group includes widespread basaltic-andesitic lava plateaus that yielded ages from 14.6 to 8.8 Ma and are interpreted as the beginning of the MVB. From 22 to 14.6 Ma, volcanic activity in the area was significantly reduced, but did not cease entirely. We refer to the third group as transitional volcanism, which is dominated by andesitic and rhyolitic lava domes but also includes high-grade andesitic ignimbrites. We conclude that the change from volcanism proper of the SMO to that of the MVB was gradual with respect to age and drastic with respect to composition and style, from a voluminous-silicic-ignimbrite domain to a widespread basaltic-andesitic-lava plateau domain. This change may have been related to major plate tectonic reorganizations within the interval from 25 to 12 Ma that involved the waning of subduction of the Farallon plate west of northern Mexico and the associated southward migration of the triple junction of the Pacific-Farallon-North America plates, the subsequent break-up of the Farrallon plate into the Guadalupe and Cocos plates, and the counterclockwise and clockwise rapid rotation of the ridge between them around 16 to 12.5 Ma.  相似文献   

5.
We show here that epithermal mineralization in the Guazapares Mining District is closely related to extensional deformation and magmatism during the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental silicic large igneous province, Mexico. Three Late Oligocene–Early Miocene synextensional formations are identified by detailed volcanic lithofacies mapping in the study area: (1) ca. 27.5 Ma Parajes formation, composed of silicic outflow ignimbrite sheets; (2) ca. 27–24.5 Ma Témoris formation, consisting primarily of locally erupted mafic-intermediate composition lavas and interbedded fluvial and debris flow deposits; (3) ca. 24.5–23 Ma Sierra Guazapares formation, composed of silicic vent to proximal ignimbrites, lavas, subvolcanic intrusions, and volcaniclastic deposits. Epithermal low-to intermediate-sulfidation, gold–silver–lead–zinc vein and breccia mineralization appears to be associated with emplacement of Sierra Guazapares formation rhyolite plugs and is favored where pre-to-synvolcanic extensional structures are in close association with these hypabyssal intrusions.Several resource areas in the Guazapares Mining District are located along the easternmost strands of the Guazapares Fault Zone, a NNW-trending normal fault system that hosts most of the epithermal mineralization in the mining district. This study describes the geology that underlies three of these areas, which are, from north to south: (1) The Monte Cristo resource area, which is underlain primarily by Sierra Guazapares formation rhyolite dome collapse breccia, lapilli-tuffs, and fluvially reworked tuffs that interfinger with lacustrine sedimentary rocks in a synvolcanic half-graben bounded by the Sangre de Cristo Fault. Deposition in the hanging wall of this half-graben was concurrent with the development of a rhyolite lava dome-hypabyssal intrusion complex in the footwall; mineralization is concentrated in the high-silica rhyolite intrusions in the footwall and along the syndepositional fault and adjacent hanging wall graben fill. (2) The San Antonio resource area, underlain by interstratified mafic-intermediate lavas and fluvial sandstone of the Témoris formation, faulted and tilted by two en echelon NW-trending normal faults with opposing dip-directions. Mineralization occurs along subvertical structures in the accommodation zone between these faults. There are no silicic intrusions at the surface within the San Antonio resource area, but they outcrop ∼0.5 km to the east, where they are intruded along the La Palmera Fault, and are located ∼120 m-depth in the subsurface. (3) The La Unión resource area, which is underlain by mineralized andesite lavas and lapilli-tuffs of the Témoris Formation. Adjacent to the La Unión resource area is Cerro Salitrera, one of the largest silicic intrusions in the area. The plug that forms Cerro Salitrera was intruded along the La Palmera Fault, and was not recognized as an intrusion prior to our work.We show here that epithermal mineralization is Late Oligocene to Miocene-age and hosted in extensional structures, younger than Laramide (Cretaceous-Eocene) ages of mineralization inferred from unpublished mining reports for the region. We further infer that mineralization was directly related to the emplacement of silicic intrusions of the Sierra Guazapares formation, when the mid-Cenozoic ignimbrite flare-up of the Sierra Madre Occidental swept westward into the study area about 24.5–23 Ma ago.  相似文献   

6.
The margin of NE China, a part of the West Pacific metallogenic belt, contains innumerable low-sulphidation mineral deposits. Gold deposits in this region can be classified into three distinct types based on geology and ore mineral paragenesis: (1) low-sulphidation epithermal silver–gold deposits, (2) low-sulphidation tellurium–gold deposits, and (3) low-sulphidation epithermal tellurium–gold deposits. Ores formed during the late Early Cretaceous and the early Late Cretaceous reflect three distinct metallogenic periods: the Fuxin Stage at 115.98 ± 0.89 Ma, the Quantou Stage at 107.2 ± 0.6 Ma or <103 Ma, and the Qingshankou or Yaojiajie Stage at < 97 Ma and 88.2 ± 1.4 Ma. The Fuxin Stage is dominated by trachyandesitic magmatism, with magmas emplaced at hypabyssal depths. In comparison, the Quantou Stage is characterized by high-K calc-alkaline, calc-alkaline, and sodic andesitic, dacitic, and rhyolitic magmatism of three different suites. The first of these is a high-K calc-alkaline andesitic magmatic suite that was accompanied by the emplacement of a calc-alkaline sodic dacite during the formation of the Ciweigou and Wufeng ore deposits. The second suite is dominated by calc-alkaline sodic rhyolite and high-K calc-alkaline sodic dacite magmatism associated with the formation of the Sipingshan ore deposit. The third suite is typified by high-K calc-alkaline andesitic magmatism associated with the emplacement of calc-alkaline hypabyssal granitoid complexes accompanying the formation of the Dong'an and Tuanjiegou ore deposits. The Qingshankou or Yaojia Stage is characterized by calc-alkaline sodic dacite magmatism associated with the formation of the Wuxing ore deposit. Metallogenesis during the Fuxin Stage characterized by trachytic magmatism is closely related to the formation of a deep-seated fault within a magmatic arc or the back-arc region of an immature continental margin and is associated with the Early Cretaceous subduction of the Pacific plate beneath Eurasia. Ore deposits that formed during the Fuxin Stage were generally related to magmato-hydrothermal fluids associated with mantle-derived magmas. In contrast, metallogenesis during the Quantou and Qingshankou or Yaojiajie stages was closely related to the formation of a mature high-K calc-alkaline magmatic arc within a continental margin setting again associated with the westward subduction of the Pacific plate. This metallogenic event was a product of magmato-hydrothermal systems derived from crust–mantle interaction and mixing of magmas derived from partial melting of different sections of the continental crust.  相似文献   

7.
In northwestern California, the Franciscan subduction complex has been subdivided into seven major tectonostratigraphic units. We report U-Pb ages of ≈2400 detrital zircon grains from 26 sandstone samples from 5 of these units. Here, we tabulate each unit’s interpreted predominant sediment source areas and depositional age range, ordered from the oldest to the youngest unit. (1) Yolla Bolly terrane: nearby Sierra Nevada batholith (SNB); ca. 118 to 98 Ma. Rare fossils had indicated that this unit was mostly 151–137 Ma, but it is mostly much younger. (2) Central Belt: SNB; ca. 103 to 53 Ma (but poorly constrained), again mostly younger than previously thought. (3) Yager terrane: distant Idaho batholith (IB); ca. 52 to 50 Ma. Much of the Yager’s detritus was shed during major core complex extension and erosion in Idaho that started 53 Ma. An Eocene Princeton River–Princeton submarine canyon system transported this detritus to the Great Valley forearc basin and thence to the Franciscan trench. (4) Coastal terrane: mostly IB, ±SNB, ±nearby Cascade arc, ±Nevada Cenozoic ignimbrite belt; 52 to <32 Ma. (5) King Range terrane: dominated by IB and SNB zircons; parts 16–14 Ma based on microfossils. Overall, some Franciscan units are younger than previously thought, making them more compatible with models for the growth of subduction complexes by progressive accretion. From ca. 118 to 70 Ma, Franciscan sediments were sourced mainly from the nearby Sierra Nevada region and were isolated from southwestern US and Mexican sources. From 53 to 49 Ma, the Franciscan was sourced from both Idaho and the Sierra Nevada. By 37–32 Ma, input from Idaho had ceased. The influx from Idaho probably reflects major tectonism in Idaho, Oregon, and Washington, plus development of a through-going Princeton River to California, rather than radical changes in the subduction system at the Franciscan trench itself.  相似文献   

8.
Magmatism in NW Mexico records a Late Miocene transformation from convergence to extension in the Gulf of California rift system. Miocene calc-alkalic rocks in the Baja California peninsula are related to the final subduction of the Farallon plate system, but the heterogeneous nature of volcanism younger than 12.5 Ma has led to conflicting tectonic interpretations. Neogene volcanic rocks in the Sierra Santa Ursula, Sonora, were emplaced in three magma pulses, according to mapping, K–Ar geochronology, and geochemistry. From 23.5 to 15 and 14 to 11.4 Ma, calc-alkalic rocks show an arc-like signature. The 12–11 Ma calc-alkalic dacites, however, are characterized by higher K, Rb, 87Sr/86Sr, and light REE abundances than are the older rocks. The timing, petrography, and geochemistry of the 12–11 Ma rocks are interpreted to reflect postsubduction magmatism. A change in magma chemistry from predominantly calc-alkalic to tholeiitic rocks at 10.3 Ma corresponds to orthogonal extension during early Gulf of California evolution. Sr, Nd, and Pb radiogenic isotope signatures show minor changes over time. The volcanic record for 20–12.5 Ma at Sierra Santa Ursula and adjacent areas is consistent with the reconstructed history of the Guadalupe microplate. The interval of magmatism produced from 12 to 11 Ma appears to reflect changes in plate geometry during the transition from subduction to rifting.  相似文献   

9.
Voluminous (3·9 x 105 km3), prolonged (18 Myr) explosivesilicic volcanism makes the mid-Tertiary Sierra Madre Occidentalprovince of Mexico one of the largest intact silicic volcanicprovinces known. Previous models have proposed an assimilation–fractionalcrystallization origin for the rhyolites involving closed-systemfractional crystallization from crustally contaminated andesiticparental magmas, with <20% crustal contributions. The lackof isotopic variation among the lower crustal xenoliths inferredto represent the crustal contaminants and coeval Sierra MadreOccidental rhyolite and basaltic andesite to andesite volcanicrocks has constrained interpretations for larger crustal contributions.Here, we use zircon age populations as probes to assess crustalinvolvement in Sierra Madre Occidental silicic magmatism. Laserablation-inductively coupled plasma-mass spectrometry analysesof zircons from rhyolitic ignimbrites from the northeasternand southwestern sectors of the province yield U–Pb agesthat show significant age discrepancies of 1–4 Myr comparedwith previously determined K/Ar and 40Ar/39Ar ages from thesame ignimbrites; the age differences are greater than the errorsattributable to analytical uncertainty. Zircon xenocrysts withnew overgrowths in the Late Eocene to earliest Oligocene rhyoliteignimbrites from the northeastern sector provide direct evidencefor some involvement of Proterozoic crustal materials, and,potentially more importantly, the derivation of zircon fromMesozoic and Eocene age, isotopically primitive, subduction-relatedigneous basement. The youngest rhyolitic ignimbrites from thesouthwestern sector show even stronger evidence for inheritancein the age spectra, but lack old inherited zircon (i.e. Eoceneor older). Instead, these Early Miocene ignimbrites are dominatedby antecrystic zircons, representing >33 to 100% of the datedpopulation; most antecrysts range in age between 20 and 32 Ma.A sub-population of the antecrystic zircons is chemically distinctin terms of their high U (>1000 ppm to 1·3 wt %) andheavy REE contents; these are not present in the Oligocene ignimbritesin the northeastern sector of the Sierra Madre Occidental. Thecombination of antecryst zircon U–Pb ages and chemistrysuggests that much of the zircon in the youngest rhyolites wasderived by remelting of partially molten to solidified igneousrocks formed during preceding phases of Sierra Madre Occidentalvolcanism. Strong Zr undersaturation, and estimations for veryrapid dissolution rates of entrained zircons, preclude coevalmafic magmas being parental to the rhyolite magmas by a processof lower crustal assimilation followed by closed-system crystalfractionation as interpreted in previous studies of the SierraMadre Occidental rhyolites. Mafic magmas were more probablyimportant in providing a long-lived heat and material flux intothe crust, resulting in the remelting and recycling of oldercrust and newly formed igneous materials related to Sierra MadreOccidental magmatism. KEY WORDS: ignimbrite; rhyolite; Sierra Madre Occidental; Tertiary; U–Pb geochronology; zircon; antecryst; crustal melting  相似文献   

10.
The gondwanan magmatism in the San Rafael Massif, known as Choiyoi Magmatic Cycle, was emplaced during the inception of a magmatic arc setting during the early Permian. Two different sections can be differentiated in this volcanic sequence. The lower section (∼281 up to ∼265 Ma) consisting of andesites and dacitic to low-silica rhyolitic ignimbrites has geochemical characteristics that indicate a subduction zone setting. The upper section (∼265 up to ∼252 Ma) composed of rhyolitic ignimbrites and lava flows, dacitic to rhyolitic subvolcanics and alkalic basaltic andesites has geochemical characteristics transitional between subduction and continental intraplate settings. Several Cu–(Mo) porphyry deposits are genetically linked to the lower section (Infiernillo, San Pedro and La Chilca-Zanjón del Buitre). In this paper, we discussed the petrogenesis of the magmatism linked to the porphyry deposits from the San Rafael Massif. The petrogenetic analysis suggests that the lower section was produced in a thickened crust resulting in an adakite-like signature magmatism. The U/Pb LA-ICP-MS age of magmatic zircons from an intrusive associated to the San Pedro porphyry (263.1 ± 4.2 Ma) allowed confirming that the emplacement of Cu–Mo porphyry deposits in the San Rafael Massif occurred during the change in the geodynamical conditions from a transpressive to a transtensive tectonic regime.  相似文献   

11.
The area of Arghash in northeast Iran, prominent for its gold mineralization, was newly mapped on a scale of 1:20,000 with particular attention to the occurring generations of igneous rocks. In addition, geochronological and geochemical investigations were carried out. The oldest geological unit is a late Precambrian, hornblende-bearing diorite pluton with low-K composition and primitive isotope signatures. This diorite (U–Pb zircon age 554 ± 6 Ma) is most likely a remnant from a Peri-Gondwana island-arc or back-arc basin. About one-third of the map area is interpreted as an Upper Cretaceous magmatic arc consisting of a volcanic and a plutonic part. The plutonic part is represented by a suite of hornblende-bearing medium-K, I-type granitoids (minor diorite, mainly quartz–monzodiorite and granodiorite) dated at 92.8 ± 1.3 Ma (U–Pb zircon age). The volcanic part comprises medium-K andesite, dacite and tuffitic rocks and must be at least slightly older, because it is locally affected by contact metamorphism through the hornblende–granitoids. The Upper Cretaceous arc magmatism in the Arghash Massif is probably related to the northward subduction of the Sabzevar oceanic basin, which holds a back-arc position behind the main Neotethys subduction front. Small occurrences of pillow basalts and sediments (sandstone, conglomerate, limestone) tectonically intercalated in the older volcanic series may be relics of earlier Cretaceous or even pre-Cretaceous rocks. In the early Cenozoic, the Cretaceous magmatic arc was intruded by bodies of felsic, weakly peraluminous granite (U–Pb zircon age 55.4 ± 2.3 Ma). Another strong pulse of magmatism followed slightly later in the Eocene, producing large masses of andesitic to dacitic volcanic rocks. The geochemistry of this prominent Eocene volcanism is very distinct, with a high-K signature and trace element contents similar to shoshonitic series (high P, Zr, Cr, Sr and Ba). High Sr/Y ratios feature affinities to adakite magmas. The Eocene magmatism in the Arghash Massif is interpreted as related to thermal anomalies in crust and mantle that developed when the Sabzevar subduction system collapsed. The youngest magmatic activities in the Arghash Massif are lamprophyres and small intrusions of quartz–monzodiorite porphyries, which cut through all other rocks including an Oligocene–Miocene conglomerate cover series.  相似文献   

12.
《International Geology Review》2012,54(13):1475-1496
We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.  相似文献   

13.
The Neogene–Quaternary volcanic products, related to Arabian and Anatolian Plate collision along the Bitlis Suture Zone, cover wide areas on both plates. One of these volcanic exposures on the Arabian Plate is the Kepez volcanic complex (KVC). This study aims explain to petrogenesis of KVC. Although some examples display alkaline affinities, the majority of the volcanic rock is calc-alkaline and can be defined in three main groups. 40Ar/39Ar data obtained from dacite, basalt and andesite rock groups within the KVC yield ages of between 13.5 and 15.5 Ma. Geochemical and petrographical data show that the andesitic rocks are products of homogeneous mixing between basic end-member magmas and dacitic magmas which are the products of partial melting of lower crustal compositions. Basaltic products of KVC are asthenospheric mantle derived, while dacitic and andesitic volcanic rocks are crustal origin. High Sr and Nd isotope ratios may indicate that andesitic and dacitic rocks originated from continental crust. The lithospheric mantle, which is subducting underneath the Anatolian plate, must have experienced slab break-off processes 13–15 million years ago and sunk into the asthenosphere. KVC were produced with the collision between Arabian and Anatolian Plates and related uplift of the East Anatolia region.  相似文献   

14.
《Geodinamica Acta》2013,26(3-4):239-253
The precise ages of the sedimentary successions within two prominent NE-SW-trending basins, the Gördes Basin and the Selendi Basin, are critical to an understanding of the Neogene evolutionary setting of western Turkey and associated calc-alkaline magmatism. Early radiometric dating was not always sufficiently precise to resolve alternative interpretations. During this study, high-precision Ar40-Ar39 radiometric ages were determined on single crystals of biotite and sanidine from silicic tuffs and associated intrusive rocks. Finegrained tuffaceous sediments near the top of the sedimentary succession in the Selendi Basin gave ages of 18.89 ± 0.58 Ma to 16.42 ± 0.09 Ma. Coarser-grained tuffaceous sediments near the top of the equivalent sedimentary succession in the Gördes Basin to the NW yielded ages of 18.78 ± 0.3 Ma to 17.04 ± 0.35 Ma. Associated intrusive rocks were dated at 20.86 ± 0.08 Ma to 17.62 ± 0.07 Ma. An andesitic body on the northern margin of the Gediz (Ala?ehir) Graben further south gave ages of 16.08 ± 10.91 to 14.65 ± 0.06 Ma.

Combined with published radiometric age data, these new results confirm an Early Miocene age for the clastic sedimentary fills of the Gördes and Selendi basins. The results from the Gediz Graben are consistent with its formation in Early Miocene time, earlier than the Late Miocene or Plio-Quaternary ages suggested in some interpretations.  相似文献   

15.
The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at a time characterized by significant changes in the plate interactions in this region as a result of the formation of the Caribbean plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. The change from SMS magmatism to an E–W trending TMVB volcanism in Miocene time reflects the tectonic evolution of southwestern Mexico during these episodes of plate tectonic rearrangement.The distribution and petrographic characteristics of the magmatic rocks of the SMS define two belts of NW orientation. The first is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks of predominantly felsic composition. The second belt is inland of the first and consists of discontinuously distributed volcanic fields with piles of andesitic to rhyolitic flows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along a continental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic differences. These basements originated during different tectono-thermal events developed from the Proterozoic to the Mesozoic.Major and trace element data of the SMS magmatic rocks define a clear sub-alkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate different degrees of magmatic differentiation and/or crustal contamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to have been controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to transtensional tectonics produced less differentiated volcanic rocks with an apparently lower degree of crustal contamination than those of northeastern Guerrero.The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate a southeastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB. SMS magmatic rocks exposed to the west of the 100°W meridian are dominantly Late Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar to that of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene magmatic rocks show a trend that roughly defines an ESE orientation. The change in the trend of arc magmatism may be the effect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The transtensional tectonic regime developed in Oligocene time in NW Oaxaca probably accentuated this trend by facilitating magma generation and ascent in these northerly regions.The geochronologic data of the SMS, in conjunction with those of the TMVB, suggest that there is a spatial and temporal magmatic gap in south central Mexico between 97 and 100°W longitude during late Oligocene and middle Miocene time (24–16 Ma). This magmatic gap is interpreted in terms of a combination of the relatively rapid change in the subducted slab geometry after the passage of the Chortis block from a moderate to a shallow angle and the time needed for the mantle wedge to mature sufficiently to produce magmas.  相似文献   

16.
New U–Pb SHRIMP zircon ages combined with geochemical and isotope investigation in the Sierra de Maz and Sierra de Pie de Palo and a xenolith of the Precordillera basement (Ullún), provides insight into the identification of major Grenville-age tectonomagmatic events and their timing in the Western Sierras Pampeanas. The study reveals two contrasting scenarios that evolved separately during the 300 Ma long history: Sierra de Maz, which was always part of a continental crust, and the juvenile oceanic arc and back-arc sector of Sierra de Pie de Palo and Ullún. The oldest rocks are the Andino-type granitic orthogneisses of Sierra de Maz (1330–1260 Ma) and associated subalkaline basic rocks, that were part of an active continental margin developed in a Paleoproterozoic crust. Amphibolite facies metamorphism affected the orthogneisses at ca. 1175 Ma, while granulite facies was attained in neighbouring meta-sediments and basic granulites. Interruption of continental-edge magmatism and high-grade metamorphism is interpreted as related to an arc–continental collision dated by zircon overgrowths at 1170–1230 Ma. The next event consisted of massif-type anorthosites and related meta-jotunites, meta-mangerites (1092 ± 6 Ma) and meta-granites (1086 ± 10 Ma) that define an AMCG complex in Sierra de Maz. The emplacement of these mantle-derived magmas during an extensional episode produced a widespread thermal overprint at ca. 1095 Ma in neighbouring country rocks. In constrast, juvenile oceanic arc and back-arc complexes dominated the Sierra de Pie de Palo–Ullún sector, that was fully developed ca. 1200 Ma (1196 ± 8 Ma metagabbro). A new episode of oceanic arc magmatism at ～1165 Ma was roughly coeval with the amphibolite high-grade metamorphism of Sierra de Maz, indicating that these two sectors underwent independent geodynamic scenarios at this age. Two more episodes of arc subduction are registered in the Pie de Palo–Ullún sector: (i) 1110 ± 10 Ma orthogneisses and basic amphibolites with geochemical fingerprints of emplacement in a more mature crust, and (ii) a 1027 ± 17 Ma TTG juvenile suite, which is the youngest Grenville-age magmatic event registered in the Western Sierras Pampeanas. The geodynamic history in both study areas reveals a complex orogenic evolution, dominated by convergent tectonics and accretion of juvenile oceanic arcs to the continent.  相似文献   

17.
Eocene to late Miocene magmatism in the central Peruvian high-plain (approx. between Cerro de Pasco and Huancayo; Lats. 10.2–12°S) and east of the Cordillera Occidental is represented by scattered shallow-level intrusions as well as subaerial domes and volcanic deposits. These igneous rocks are calc-alkalic and range from basalt to rhyolite in composition, and many of them are spatially, temporally and, by inference, genetically associated with varied styles of major polymetallic mineralization. Forty-four new 40Ar–39Ar and three U/Pb zircon dates are presented, many for previously undated intrusions. Our new time constraints together with data from the literature now cover most of the Cenozoic igneous rocks of this Andean segment and provide foundation for geodynamic and metallogenetic research.The oldest Cenozoic bodies are of Eocene age and include dacitic domes to the west of Cerro de Pasco with ages ranging from 38.5 to 33.5 Ma. South of the Domo de Yauli structural dome, Eocene igneous rocks occur some 15 km east of the Cordillera Occidental and include a 39.34 ± 0.28 Ma granodioritic intrusion and a 40.14 ± 0.61 Ma rhyolite sill, whereas several diorite stocks were emplaced between 36 and 33 Ma. Eocene mineralization is restricted to the Quicay high-sulfidation epithermal deposit some 10 km to the west of Cerro de Pasco.Igneous activity in the earliest Oligocene was concentrated up to 70 km east of the Cordillera Occidental and is represented by a number of granodioritic intrusions in the Milpo–Atacocha area. Relatively voluminous early Oligocene dacitic to andesitic volcanism gave rise to the Astabamba Formation to the southeast of Domo de Yauli. Some stocks at Milpo and Atacocha generated important Zn–Pb (–Ag) skarn mineralization. After about 29.3 Ma, magmatism ceased throughout the study region. Late Oligocene igneous activity was restricted to andesitic and dacitic volcanic deposits and intrusions around Uchucchacua (approx. 25 Ma) and felsic rocks west of Tarma (21–20 Ma). A relationship between the Oligocene intrusions and polymetallic mineralization at Uchucchacua is possible, but evidence remains inconclusive.Widespread magmatism resumed in the middle Miocene and includes large igneous complexes in the Cordillera Occidental to the south of Domo de Yauli, and smaller scattered intrusive centers to the north thereof. Ore deposits of modest size are widely associated with middle Miocene intrusions along the Cordillera Occidental, north of Domo de Yauli. However, small volcanic centers were also active up to 50 km east of the continental divide and include dacitic dikes and domes, spatially associated with major base and precious metal mineralization at Cerro de Pasco and Colquijirca. Basaltic volcanism (14.54 ± 0.49 Ma) is locally observed in the back-arc domain south of Domo de Yauli approximately 30 km east of the Cordillera Occidental.After about 10 Ma intrusive activity decreased throughout Central Perú and ceased between 6 and 5 Ma. Late Miocene magmatism was locally related to important mineralization including San Cristobal (Domo de Yauli), Huarón and Yauricocha.Overall, there is no evidence for a systematic eastward migration of the magmatic arc through time. The arc broadened in the late Eocene to early Oligocene, and thereafter ceased over wide areas until the early Miocene, when magmatism resumed in a narrow arc. A renewed widening and subsequent cessation of the arc occurred in the late middle and late Miocene. The pattern of magmatism probably reflects two cycles of flattening of the subduction in the Oligocene and late Miocene. Contrasting crustal architecture between areas south and north of Domo de Yauli probably account for the differences in the temporal and aerial distribution of magmatism in these areas.Ore deposits are most abundant between Domo de Yauli and Cerro de Pasco and were generally emplaced in the middle and late Miocene during the transition to flat subduction and prior to cessation of the arc. Eocene to early Oligocene mineralization also occurred, but was restricted to a broad east–west corridor from Uchucchacua to Milpo–Atacocha, indicating a major upper-plate metallogenetic control.  相似文献   

18.
The super-large Shuangjianzishan Pb–Zn–Ag deposit is a newly discovered deposit located in the Huanggang–Ganzhuermiao polymetallic metallogenic belt of Inner Mongolia, NE China. The deposit's resource includes 0.026 Mt Ag, 1.1 Mt Pb, and 3.3 Mt Zn. The deposit is controlled by a NW-trending ductile shear zone and NE- and NW-trending faults in black pelite assigned to the lower Permian Dashizhai Formation. LREE enrichment, HREE depletion, Nb, Ta, P, and Ti depletion, and Zr and Hf enrichment characterize felsic magmatic rocks in the Shuangjianzishan Pb–Zn–Ag district. The ages of porphyritic monzogranite, rhyolitic crystal–vitric ignimbrite, and porphyritic granodiorite are 254–252, 169, and 130 Ma, respectively. Pyrite sampled from the mineralization has Re–Os isochron ages of 165 ± 7 Ma, which suggest the mineralization is associated with the ca. 169 Ma magmatism in the Shuangjianzishan district.Zircons extracted from the porphyritic granodiorite yield εHf(t) values of − 11.34 to − 1.41, with tDM2 dates of 1275–1901 Ma. The εHf(t) values of zircons in the rhyolitic crystal–vitric ignimbrite and the ore-bearing monzogranite porphyry are 7.57–16.23 and 10.18–15.96, respectively, and their tDM2 ages are 177–733 and 257–632 Ma, respectively. Partial melting of depleted mantle resulted in the formation of the ca. 254–252 Ma ore-bearing porphyritic monzogranite and the ca. 169 Ma rhyolitic crystal–vitric ignimbrite; dehydration partial melting of subducted oceanic crust resulted in the formation of the ca. 130 Ma porphyritic granodiorite. The porphyritic monzogranite was emplaced during the late stages of closure of the Paleo-Asian Ocean during the transformation from a collisional to extensional tectonic setting. The ca. 170 and ca. 130 Ma magmatism and mineralization in the Shuangjianzishan district are related to subduction of the Mongolia–Okhotsk Ocean and subduction of the Paleo-Pacific Ocean Plate, respectively.  相似文献   

19.
 Isotopic and trace element data from mantle and granulite xenoliths are used to estimate the relative contributions of mantle and crustal components to a large ignimbrite, referred to as the upper ignimbrite, that is representative of the voluminous mid-Cenozoic rhyolites of northwestern Mexico. The study also uses data from the volcanic rocks to identify deep crustal xenoliths that are samples of new crust created by the Tertiary magmatism. The isotopic composition of the mantle component is defined by mantle-derived pyroxenites that are interpreted to have precipitated from mid-Cenozoic basaltic magmas. This component has ɛNd≈+1.5, 87Sr/86Sr≈0.7043 and 206Pb/204Pb≈18.6. Within the upper ignimbrite and associated andesitic and dacitic lavas, initial 87Sr/86Sr is positively correlated with SiO2, reaching 0.7164 in the ignimbrite. Initial 206Pb/204Pb ratios also show a positive correlation with silica, whereas ɛNd values have a crude negative correlation, reaching values as low as −2. Of the four isotopically distinct crustal components identified from studies of granulite xenoliths, only the sedimentary protolith of the paragneiss xenoliths can be responsible for the high initial 87Sr/86Sr of the upper ignimbrite. The Nd, Sr, and Pb isotopic compositions of the upper ignimbrite can be modeled with relatively modest assimilation (≤20%) of the sedimentary component ± Proterozoic granulite. Gabbroic composition granulite xenoliths have distinctive Nd, Sr, and Pb isotope ratios that cluster closely within the range of compositions found in the andesitic and dacitic lavas. These mafic granulites are cumulates, and their protoliths are interpreted to have precipitated from the intermediate to silicic magmas at 32–31 Ma. These mafic cumulate rocks are probably representative of much of the deep crust that formed during mid-Cenozoic magmatism in Mexico. Worldwide xenolith studies suggest that the relatively great depth (≤20 km) at which assimilation-fractional crystallization took place in the intermediate to silicic magma systems of the La Olivina region is the rule rather than the exception. Oligocene ignimbrites of the southwestern United States (SWUS) have substantially lower ɛNd values (e.g. <−6) than the upper ignimbrite and other rhyolites from Mexico. This difference appears to reflect a greater crustal contribution to ignimbrites of the SWUS, perhaps due to a higher temperature of the lower crust prior to the emplacement of the Oligocene basaltic magmas. Received: 16 December 1994 / Accepted: 13 September 1995  相似文献   

20.
The Strona-Ceneri Zone comprises a succession of polymetamorphic, pre-Alpidic basement rocks including ortho- and paragneisses, metasedimentary schists, amphibolites, and eclogites. The rock pile represents a Late Proterozoic or Palaeozoic subduction accretion complex that was intruded by Ordovician granitoids. Eclogites, which occur as lenses within the ortho-paragneiss succession and as xenoliths within the granitoids record a subduction related high-pressure event (D1) with peak metamorphic conditions of 710 ± 30 °C at 21.0 ± 2.5 kbar. After isothermal uplift, the eclogites experienced a Barrowtype (D2) tectonometamorphic overprint under amphibolite facies conditions (570-630 °C, 7-9 kbar). U-Pb dating on zircon of the eclogites gives a metamorphic age of 457 ± 5 Ma, and syn-eclogite facies rutile gives a 206Pb/238U age of 443 ± 19 Ma classifying the subduction as a Caledonian event. These data show that the main tectonometamorphic evolution of the Strona-Ceneri Zone most probably took place in a convergent margin scenario, in which accretion, eclogitization of MOR-basalt, polyphase (D1 and D2) deformation, anatexis and magmatism all occurred during the Ordovician. Caledonian high-pressure metamorphism, subsequent magmatism and Barrow-type metamorphism are believed to be related to subduction and collision within the northern margin of Gondwana. Editorial handling: Edwin Gnos  相似文献   

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