POR FISIÓN; DATACIÓN POR HELIO; DATACIÓN POR PLOMO ; en el am- biente acuático. datación por potasio-argón Método para determinar la edad. Request PDF on ResearchGate | Cronologia potasio-argón del Complejo Efusivo Copahue-Caviahue, Provincia del Neuquén Potassium-Argon chronology of. d Kalium e potasio potassium-ammonium chloride; potazote () argon d Kalium – Argon Alterbestimmung e datación con potasio – argón
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Ranson 1Louis A.
Triassic metasediments, a Cretaceous intrusive, and Tertiary intrusive rocks comprise the major rock types west of the city of Zacatecas. The metasediments are principally phyllites, containing conformable lenses of quartzite and marble, and fine-grained, ammonite-bearing quartzites. A detailed stratigraphic and structural study of the phyllites suggests that two episodes of deformation and accompanying metamorphism affected these rocks during Early Jurassic and Late Cretaceous time.
The metasediments are intruded by a shallow diorite pluton. Hypogene mineralization within the metasediment, the diorite, and the intrusive rhyolite appear to arfon occurred not long after the intrusion of the rhyolite. Subsequent erosion of metasediment and diorite resulted in the accumulation of a calcite- and hematite-cemented conglomerate.
Magmatism continued into the Late Tertiary with the extrusion of rhyolite flows, tuffs, and volcanic breccias. All metamorphic rocks exposed in the area of study are of the greenschist facies of regional metamorphism.
Chemical and dataccion features of the metamorphic rocks indicate that a relatively dry system may not have allowed relic sedimentary minerals to enter into metamorphic reactions, thus resulting in disequilibrium assemblages. The clase association in time of the metasediments and the diorite intrusion suggests that the temperatures necessary for greenschist facies metamorphism were attained at shallow depths in the crust. The Sierra de Zacatecas has been the site of mineral explotation since the mid’s, yet much remains to be learned about the geologic history of this region.
Datación potasio-argón (K-Ar)
This history is complicated by sparse exposure, folding and faulting of the metamorphic terrain, hydrothermal and deuteric alteration of the igneous rock, and at least one episode of ore mineralization.
The objective of this study is to attempt to unravel the geologic history of the region in the light of new geological, petrological, and geochemical data. The region of study, approximately 5 km wide and 7 km long, is located in the southern portion of the Zacatecas Quadrangle, Iocated in the State of Zacatecas. Occurring within this area are Mesozoic igneous and metamorphic rocks and Cenozoic igneous and sedimentary rocks. The metamorphic rocks are predominantly phyllites with lesser amounts of Quartzite, metaconglomerate, and marble.
Diorite and rhyolite constitute the igneous rocks. The only sedimentary rock exposed in the region is a red conglomerate composed chiefiy of metamorphic and dioritic rock fragments. The Cenozoic igneous and sedimentary rocks, and the economic geology of the Zacatecas region have been the subject of two recent Master’s theses at the University of New Orleans Brown, and Barr, In light of this, this report will concentrate of the Mesozoic igneous and metamorphic rocks ofthe region.
Datación Potasio-Argón (K-Ar)
Geologic mapping began with reconnaissance studies in January of ; but most of the mapping was accomplished during the months potsio June, JuIy, and August, Few geologic studies have been made of the Sierra de Zacatecas in spite of the great economic importance of the area.
Prior to the late ‘s, published works consisted mainIy of reconnaissance studies dealing with the entire State of Zacatecas, with only brief mention being made of the region in the vicinity of the city of Zacatecas Burkart, ; Amador, ; Burckhardt, ; Flores ; Bastin, Burckhardt and Scalia were first to publish a modern geologic map ofpart of the Sierra de Zacatecas. A report by Mapes Vasquez relics heavily on this early work and is concerned principally with the ore mineralization.
Both publications are concerned more with the ore mineralization than with the structural and petrological nature of the rocks of the region. The area of study is part of the Mesa Central morphotectonic province of central Mexico.
The eastern limit of the Mesa Central is marked by the largest morphotectonic province in Mexico, the Sierra Madrc Oriental, which extends from the United States border to the Guatemala border. This province consists mainly of a basal sequence of andesitic to basaltic volcanic rocks overlain by dacitic and rhyolitic ignimbrites, tuffs, and lavas. Morphotectonic units of Mexico after Dengo, Previous investigators of the Sierra de Zacatecas have described the rocks in the area in a general manner, and formational ilames, with one exception, have not becn applied.
Thus the formational names used in this report are being presented for the first time. Since the rhyolites are much more extensive argom of this area and are the focus of another study Brown, op. Within the phyIlite are conformable lenticular bodies of metaconglomerate, quartzite, and marble. Locally, the phyllite is tightly folded, with the fold axes trending to the northwest. This formation is the oldest unit in the fleld area. A detailed satacion sequence present in the La Pimienta Phyllite is discussed below.
The stratigraphy of the La Pimienta Phyllite is complicated by folding and shearing, lithologic variation and paucity of outcrop. The topographic map, which served as a base for the general geologic map of the region, has a scale too small 1: Poasio enlarged map of the major arroyos shows the structural detail at a scale of 1;2, Figure 2. For reasons of simplicity and clarity, the stratigraphic units which were recognized in the La Pimienta Phyllite are designated by letters rather than names.
Since there are almost no geopetal structures, the beds argn assumed to be right side up and the age relations are based purely on stratigraphic position. Thicknesses were estimated with a five foot Jacob staff. Since contacts between some units are gradationaI and the structures complicated, thicknesses are only approximate.
Color designations are taken from the RockColor Chart of Goddard et al. Summary of stratigraphic units defined in the La Pimienta Phyllite. The lowermost metamorphic unit, which is exposed in Arroyo del Alamo, is designated Unit A. Quartz and sericite compose approximately 50 and 40 percent of the rocks respectiveIy, with minor tale, argoon, and epidote accounting for the remaining 10 percent. Despite the well-developed bedding-pIane cleavage of the rocks in this unit, outcrops appear blocky when viewed from a distance of several meters.
K–Ar dating – Wikidata
The strikes of bedding-plane cleavage range from northwesterly to northeasterIy, but the angle of dip is consistently shallow. These shallow dips coupled with the fact that the Arroyo runs along strike account for the great extent of this unit. The estimated thickness of Unit A is 30m.
Above the Iowermst unit in the section is Unit B, which is composed of three distinct members, designated i, i i, and i i i, with a total thickness of m. Member i, is approximately m thick and is the most widespread member. The dtaacion is characterized by uniform texture and mineralogy, and very fine grain size 0. Quartz and sericite are present in about equal proportions and constitute 90 percent of the rock with accesory epidote 1 percentalkali feldspar 5 pltasio 7 percentand opaque minerals 2 to 4 percent.
Member ii, with a thickness of only 25 mm, is the least extensive of the members in Unit B.
This member is also a very fine-grained 0. Quartz 50 percent and sericite 40 percent are the major mineraIs, with minor alkali feldspar 6 to 7 percentepidote 1 percentand chlorite 75 percent. Member iii, which is 45 m thick, is one of the most distinctive of all the metamorphic units and is best exposed at the ends of Arroyo Del Pogasio and Arroyo Talamantes where they join to form Arroyo El Bote Figure 2.
Alternating thin layers of metaquartzite and phyllite make this member distinctive. The thickness of these layers ranges from 1 to ptasio cm. Associated quartzite and phyllite Iayers usually have the same thickness. There is a definite grain-size gradation in one direction between the layers with the quartzite becoming finer grained and more micaceous pofasio the the Iying phyllite layers; whereas the contact between the underlying phyllite and adtacion is sharp. Locally, relict graded bedding is useful in determing the tops of the beds.
The quartzite lacks obvious follation, but exhibits a potsio texture and is very hard. The amount of sericite inereases and the grain size of the quartz decreases as the quartzite grades into the phyllite. In general, this quartzite is nearly identical to the lenticular bodies of quartzite which occur in overlying units. Bedding-plane cleavage and phyllitic sheen are prominent features, argoj the grain size averages 0.
Quartz 45 percent and sericite 45 percent are the predominant minerals with accessory aIkali feldspar 3 to 5 percentepidote 1 percentand lithic fragments 4 to 6 percent. Small-scale folds characterize exposures of this member and structural measurements indicate a complex history of deformation. In general, the bedding-plane cleavage strikes to the northeast and dips at moderate angles to both the southeast and adtacion.
Pods and lenses of metaconglomerate, which are characteristic of member iii, occur both conformably and in fault contact with the interbedded phyIlite and quartzite. The metaconglomerate lenses range in size from slightly potzsio than 1 m to as much as 10m across.
The largest and best exposured outcrop of metaconglomerate appears to be in fault contact with the interbedded quartzite and phyllite. These multisized clasts are bound together by a fine grained micaceous cement. Both small and large particles are flattened or elongated such that the long axes of all grains are subparalIel.
Eighty to ninety percent of the clasts composing the metaconglomerate are quartzite, with the remaining ten to twenty percent being fragments of phyllite. Quartzite pebbles are rounded to well rounded, and phyllite fragments, which exhibit foliation, are rounded. Overlying Unit 8 are the multicolored phyllites that comprise Unit C. Both the grayish-purple argom the grayish-orange phyIlite possess bedding-plane cleavage and consists of very fine-grained 0. Minor chlorite 3 percent and epidote potaiso percent also argob present.
The grayish-purple phyllite typically contains remnant burrows. An extreme example of this burrow motling is a phyllite in which spots, averaging 3 mm in length, dot the rock. These fragments are arranged with their long axes subparallel and are associated with rounded grains of quartzite.
Both types of clasts range in size from 2 mm to 30 mm, the average size being about 10 mm. Interstitial to these detrital clasts are secondary quartz and mixtures of fine-grained quartz and sericite, which bind the grains together.
The abundance of phyllite fragments readily differentiates this metaconglomerate from that found in Unit B. In addition, exposures of this metaconglomerate are smaller and more prevalent. Lenses of marble with exposures ranging in size from a few square meters to approximately 20 m 2 occur conformably within the phyllite of Unit C. The marble is dominantly composed of very fine-grained 0. This very ragon calcite may coarsen to blocky, sparry calcite, and veins ofsecondary sparry calcite potassio abundant.
Locally the marble is intensely folded and foliated Figure 3.
At most exposures, however, the marble appears massive and unfolded. Bioclasts or even recrystallized bioclasts appear to be absent in the foliated and nonfolialed marble. Unit D is unique in its appearance and limited extent.
The rock has bedding-plane cleavage and fine banding parallel potaxio this cleavage. Aggregates ofquartzofeldspathic minerals, as large as 1 cm in length, conslitute the white portions of the rock.