纳长石花岗岩中雪球结构形成机理的研究

在某些富锂、氟含稀有金属花岗岩的石英和钾长石斑晶中常见（半）雪球结构。雪球结构的产出特征、雪球体中钠长石的电子探针分析结果以及其他间接证据都说明,雪球结构是在岩浆结晶分异过程中形成的。对矿物结晶顺序、石英和纳长石的生长速率以及固相线温度的研究表明,富锂、氟、钠的花岗质残余岩浆完全具备形成雪球结构的条件．岩浆熔体中较高的Ｎａ２Ｏ／Ｋ２Ｏ比值和Ｆ、Ｈ２Ｏ含量在雪球结构的形成过程中起着重要的作用：Ｆ的高     

钠长石花岗岩中雪球结构形成机理的研究

在某些富锂、氟含稀有金属花岗岩的石英和钾长石斑晶中常见 (半 )雪球结构。雪球结构的产出特征、雪球体中钠长石的电子探针分析结果以及其他间接证据都说明 ,雪球结构是在岩浆结晶分异过程中形成的。对矿物结晶顺序、石英和钠长石的生长速率以及固相线温度的研究表明 ,富锂、氟、钠的花岗质残余岩浆完全具备形成雪球结构的条件。岩浆熔体中较高的Na2 O/K2 O比值和F、H2 O含量在雪球结构的形成过程中起着重要的作用 :F的高含量使岩浆固相线温度降低 ,岩浆得以充分分异演化 ,形成接近端员组分的钾长石和钠长石 ;Na2 O/K2 O比值较大使钠长石首先结晶 ;较高的F和H2 O含量使岩浆粘度降低 ,石英的生长速率相对加快并逐渐包裹钠长石形成雪球结构。     

钠长花岗岩—H2O—HF体系中流体／熔体间氟的分配实验研究

在ｐ＝１００Ｍｐａ,在７７０℃≤ｔ≤８００℃和ＷＦ＝２％－６％条件下进行了钠长花岗岩－Ｈ２Ｏ－ＨＦ体系液相线附近线体／熔体间氟的分配实验。对淬火玻璃的主要元素和氟含量进行了电子探针测定。用质量平衡法计算了流体中的氟含量,所获得的氟的流体／熔体分配系数ＤＦ均不盱１．０随体系氟含量的增加ＤＦ有所增大,表明在花岗岩岩浆一热液体系,氟优先进入熔体相,含氟花岗岗岩结晶晚期流体释放阶段残余熔体仍保保持富氟特征     

胶东台上金矿成矿机制模拟实验研究

模拟实验研究表明，台上金矿床的成矿机制为：胶东群地层重熔形成滦家河花岗岩岩浆时，初始浆为几乎均匀的熔体相；随着结晶成岩作用的进行，由于局部岩浆结晶分异作用，产生富Ｋ＾＋，Ｎａ＾２＾＋，Ｃａ＾２＾＋，Ｍｇ＾２＾＋，Ｆ＾－，Ｃｌ＾－，Ｓ＾２＾－，ＣＯ３＾２＾－及挥发份ＣＯ２，ＣＯ，Ｈ２Ｏ等的流体相，并不断从已结晶矿物及残熔体中，以［Ａｕ（ＨＳ）２］＾－，［Ａｕ（ＨＳ）Ｓ］＾２＾－，ＡｕＣｌ４］＾－等络     

岚山头一带含霓石碱长花岗岩的基本特及其地质意义

岗山头一带含霓石碱长花岗岩分布在胶南造山带的南段，是华北板块与扬子板块碰撞晓期在引张环境下形成的Ａ型花岗岩。该花岗岩含有霓石、钠闪石、微斜长石、钠长石等碱性矿物；Ｋ２Ｏ和Ｎａ２Ｏ的含量为８．４％－９．６％，Ｈ２Ｏ＋少于０．１６％，具有富碱，贫水的特点。     

江西凤凰岽含锡黄英岩脉特征及成因

江西会昌县岩背火山盆地密坑山含黄玉花岗岩体的北侧外接触带，产有数条含锡黄英岩脉。据野外产状、岩石结构构造等表明岩脉属岩浆成因。稀土元素研究表明，凤凰岽黄英岩ＲＥＥ含量极低，∑ＲＥＥ及配分曲线与脉侧黄玉石英化蚀变岩有着明显的区别。富氟花岗岩浆结晶晚期衍生的富Ｓｉ，Ａｌ，Ｆ含少量Ｋ，Ｎａ的残余熔浆沿断裂上侵因压力降低产生二次沸腾，使Ｋ，Ｎａ分离进入含水相，这是富Ｓｉ，Ａｌ，Ｆ；极贫Ｋ，Ｎａ的黄英岩形成的决定因素。     

岩浆熔体中的挥发性组分及其岩石学意义

文章论述了挥发性组分在岩浆熔体中的溶解行为及其在岩浆熔体中溶解的限制条件，在此基础上讨论了挥发性组分对岩浆演化及岩石学成因方面的意义：溶解在岩浆中的水可明显地改变矿物从岩浆中结晶出的顺序，改变岩浆熔体的演化轨迹。含水玄武岩浆在其早期结晶出铁的氧化物并与Ｓｉ和富碱质组分分异从而使其成为Ｆｅ亏损的钙－碱性系列的岩浆，而无水玄武岩浆演化的早期不能结晶出磁铁矿，因而演化成富Ｆｅ且具中等Ｓｉ含量的拉斑玄武岩系列。溶解于岩浆中Ｈ２Ｏ的离解将产生高ｆＯ２环境，从而导致Ｆｅ的氧化物在岩浆演化早期沉淀。     

花岗岩—KBF4—Na2MoO4—WO3体系的实验研究及其矿床学意义

为探讨长英质岩浆作用过程中金属成矿元素的地球化学行为及其成矿意义，我们进行了常压下花岗岩－ＫＢＦ３－Ｎａ２ＭｏＯ４－ＷＯ３体系的实验研究。结果表明，高温（１２５０℃）条件下呈均一状态的花岗岩－ＫＢＦ４－ＮａＭｏＯ４－ＷＯ３体系，当温度降低时发生液态不混溶，从中分离出含矿熔体的小液滴，体系中的Ｍｏ（Ｗ）几乎全部富集在这种小液滴中。含矿熔体中极富含Ｃａ、Ｍｇ和Ｐ，而贫Ｓｉ、Ａｌ和Ｋ，Ｈ２Ｏ和Ｆ富集在含矿熔体中。此实验结果表明：长英质岩浆中液态不混溶作用的发生可以使成矿元素Ｗ和Ｍｏ富集到与硅酸盐熔体不混溶的独立的非硅酸盐熔体中。这种熔体在适当的地质条件下继续演化可形成类似镁铁质岩浆演化过程中常出现的岩浆熔离型矿床。本实验结果可能为斑岩矿床的形成机理提供一种新的解释。     

花岗岩－KBF＿4－Na＿2MoO＿4体系的实验结果

花岗岩－ＫＢＦ＿４－Ｎａ＿２ＭｏＯ＿４体系的实验结果朱永峰（北京大学地质学系，１００８７１）常压下花岗岩－ＫＢＦ＿４－Ｎａ＿２ＭｏＯ＿４体系在１２５０℃条件下呈均匀熔体状态，成矿元素Ｍｏ及其它元素（如Ｆ，Ｃｌ，Ｈ＿２Ｏ及变网离子Ｃａ，Ｎａ，Ｋ，Ｍｇ等...     

长英质岩浆中液态不混溶与成矿作用关系的实验研究

本文的实验研究表明：１２５０℃（１０５Ｐａ）条件下呈均一状态的花岗岩－ＫＢＦ４－Ｎａ２ＭｏＯ４体系，在１０００℃条件下发生液态不混溶形成三种熔体：相对偏酸性的液滴、相对偏基性的熔体和成矿熔体。成矿熔体中富含ＣａＯ、ＭｇＯ和ＭｏＯ２组分。红外光谱研究表明：硅酸盐熔体结构以［ＳｉＯ４］基团为主，而成矿熔体结构中存在Ｃａ—Ｆ、Ｃａ—Ｏ—Ｍｏ、Ｈ—Ｏ—Ｈ以及Ｘ─ＯＨ（Ｘ＝阳离子）基团，说明Ｈ２Ｏ和Ｆ富集在成矿熔体中。液态不混溶作用产生的三种不混溶熔体的结构差异明显，主要表现在结构单元中桥氧数目的变化，相对偏酸性的Ａ类小球体的结构单元中桥氧数最高，基底玻璃熔体中桥氧数则相对较少，而成矿熔体中不存在桥氧。本实验研究结果表明长英质岩浆中的液态不混溶可直接导致成矿熔体的形成。这意味着长英质岩浆中的液态不混溶可直接导致斑岩矿床的形成。     

Textural evidence of magma decompression, devolatilization and disequilibrium quenching: an example from the Western Krušné hory/Erzgebirge granite pluton

We report new occurrences of “two-phase” granitic textures from the Western Krušné hory/Erzgebirge pluton (central Europe) and use crystal-size distribution data and thermodynamic modeling to interpret their crystallization conditions. The two-phase texture consists of (1) early phenocrysts of quartz, plagioclase, K-feldspar and biotite, (2) medium-grained matrix of the same phases and (3) interstitial channels and patches of a late-stage, very fine-grained matrix. The porphyritic two-mica microgranites, which host two-phase textures, occur as minor intrusions in early low-F biotite granites or as marginal parts of evolved high-F Li-mica granites. Measurements of the crystal-size distribution of quartz revealed three grain populations: (1) early phenocrysts (0.5–3.0 mm) showing partial resorption by residual melt, (2) a medium-grained population of the equigranular rock matrix (0.05–0.50 mm) that experienced minor coarsening by subsolidus annealing and (3) a fine-grained population (<0.03 mm) in the interstitial channels and patches formed during rapid devolatilization; this quartz group shows no or poor grain coarsening. All samples exhibit similar fraction of the fine-grained population (44–52%) but proportions of phenocrysts to medium-grained matrix vary significantly. Thermodynamic modeling of liquidus equilibria and experimental data in the hydrous haplogranite system require: (1) ascent of a granitic suspension (15–25% phenocrysts) under H₂O-undersaturated conditions at 25–45 bar/°C and a cooling rate of 40 J/(g kbar) in order to produce partial resorption of quartz phenocrysts and continued growth of feldspar phenocrysts, followed by (2) emplacement as discrete intrusions or bodies along pluton roof accompanied by sudden devolatilization. At the onset of matrix nucleation, disequilibrium undercooling of 70–85°C was inferred from the presence of micrographic intergrowths of quartz and K-feldspar. The two-phase granites in the Western Krušné hory/Erzgebirge pluton and in the Southeast Asian batholith form compositionally narrow groups with high-silica and moderate volatile enrichments but they differ in peraluminosity and phosphorus concentrations. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Application of cathodoluminescence to magmatic quartz in a tin granite – case study from the Schellerhau Granite Complex, Eastern Erzgebirge, Germany

A model of the cooling history of tin-bearing granitic magma forming the Schellerhau granites (Eastern Erzgebirge, Germany) is shown on the basis of quartz textures. Similar grain size, similar grain habit and correlatable growth textures of phenocrysts in different granite varieties give proof of a common crystallization history before the melts of the Schellerhau granite varieties were intruded. Four nucleation events occurred during crystallization in different crustal levels between about 20 and 1 km depth. The parental melt of the Schellerhau granites is interpreted to have contained<2.5 wt% H₂O originally. The water content of the melt during the subvolcanic intrusion stage amounted to more than 5 wt% and characterizes highly evolved residual melts that enable the formation of tin deposits. This study contributes to a better understanding of the development and behaviour of fractionated tin-bearing granitic melts, and links quartz cathodoluminescence (CL) with microanalytical studies. Received: 28 October 1998 / Accepted: 18 August 1999     

Significance of selective crystal entrainment and differential crystal-melt separation in petrogenesis of granites from the Tongbai orogen

Partial melting has been shown to be an important mechanism for intracrustal differentiation and granite petrogenesis. However, a series of compositional differences between granitic melt from experiments and natural granites indicate that the processes of crustal differentiation are complex. To shed light on factors that control the processes of crustal differentiation, and then the compositions of granitic magma, a combined study of petrology and geochemistry was carried out for granites (in the forms of granitic veins and parautochthonous granite) from a granulite terrane in the Tongbai orogen, China. These granites are characterized by high SiO₂ (>72 wt%) and low FeO and MgO (<4 wt%) with low Na₂O/K₂O ratios (<0.7). Minerals in these granites show variable microstructures and compositions. Phase equilibrium modelling using P–T pseudosections shows that neither anatectic melts nor fractionated melts match the compositions of the target granites, challenging the conventional paradigm that granites are the crystallized product of pure granitic melts. Based on the microstructural features of minerals in the granites, and a comparison of their compositions with crystallized minerals from anatectic melts and minerals in granulites, the minerals in these granitoids are considered to have three origins. The first is entrained garnets, which show comparable compositions with those in host granulites. The second is early crystallized mineral from melts, which include large plagioclase and K-feldspar (with high Ca contents) crystals as well as a part of biotite whose compositions can be reproduced by crystallization of the anatectic melts. The compositions of other minerals such as small grained plagioclase, K-feldspar and anorthoclase in the granites with low Ca contents are not well reconstructed, so they are considered as the third origin of crystallized products of fractionated melts. The results of mass balance calculation show that the compositions of these granites can be produced by mixing between different proportions of crystallized minerals and fractionated melts with variable amounts of entrained minerals. However, the calculated modal proportions of different crystallized minerals (plagioclase, K-feldspar, biotite and quartz) in the granites are significantly different from those predicted by melt crystallization modelling. Specifically, some rocks have lower modes of biotite and plagioclase, whereas others show lower K-feldspar modes than those produced by melt crystallization. This indicates that the crystallized minerals would be differentially separated from the primary magmas to form the evolved magmas that produce these granites. Therefore, the crystal entrainment and differential melt-crystal separation make important contributions to the composition of the target granites. Compared with leucogranites worldwide, the target granites show comparable compositions. As such, the leucogranites may form through the crystal fractionation of primary granitic magmas at different extents in addition to variable degrees of partial melting.     

松潘—甘孜造山带万里城花岗岩及其岩浆包体的成因与地球动力学意义

松潘—甘孜造山带广泛分布着三叠纪花岗岩体,其成因对正确认识研究区花岗岩浆的动力学背景具有重要意义。地球化学分析表明,万里城岩体寄主花岗岩具有高的SiO2含量（69.43%~73.10%）和较高的全碱含量,具弱过铝质（A/CNK=1.01~1.12）特征,属于高钾钙碱性—钾玄岩系列I型花岗岩类。暗色微粒包体具较低的SiO2含量（52.85%~59.50%）和较高的Mg#值（45~63）,为准铝质高钾钙碱性二长（闪长）岩。包体为典型的岩浆细粒结构,发育针状磷灰石、环带结构斜长石、瞳状石英、反鲍文序列的不平衡岩浆结构等。微量与稀土元素分析表明,包体起源于壳幔混合作用,是底侵的幔源玄武质岩浆与上覆壳源长英质岩浆混合的产物,混合的熔体经历了钛铁矿、黑云母等矿物的分离结晶,最终形成万里城暗色微粒包体。而寄主花岗岩则起源于纯的长英质陆壳,岩石具有较低的Mg#值(21~39)、中等的CaO/(MgO+TFeO)值、较高的K2O/Na2O和(Na2O+K2O)/(TFeO+MgO+TiO2)值等,指示源区主要为变杂砂岩类。综合区域地质资料,提出松潘—甘孜造山带内大规模花岗质岩体的形成主要受控于碰撞后伸展背景下的玄武质岩浆底侵加热。     

Muscovite dehydration melting: Reaction mechanisms,microstructures, and implications for anatexis

Dehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K⁺–Na⁺ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7–8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H₂O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.     

Contrasting evolution of low-P rare metal granites from two different terranes in the Hoggar area, Algeria

Two mineralogically different rare metal granites located in two distinct terranes from the Tuareg area are compared: the Tin-Amzi granite in the north of the Laouni Terrane and the Ebelekan granite in the Assodé–Issalane Terrane.The Tin-Amzi granite is enclosed within Eburnean granulitic gneisses, and consists of albite, quartz, protolithionite, K-feldspar and topaz granite (PG). The accessory minerals include columbite tantalite, U- and Hf-rich zircon, Th-uraninite, wolframoixiolite and wolframite. This facies is characterised by a mineralogical evolution from the bottom to the top underlined by a strong resorption of K-feldspar and albite and the crystalliK-feldspar of more abundant topaz and protolithionite II which is further altered in muscovite and Mn-siderite. It is underlain by an albite, K-feldspar, F-rich topaz, quartz and muscovite granite (MG), with W–Nb–Ta oxides, wolframite, Nb-rutile, zircon and scarce uranothorite as accessories.The Ebelekan granite intrudes into a coarse-grained biotite granite enclosed within upper amphibolite-facies metasediments. It comprises a zinnwaldite, albite, topaz porphyritic granite (ZG) with “snow ball” quartz and K-feldspar. The accessories are zircon, monazite, uranothorite, Ta bearing cassiterite, columbite tantalite and wodginite. It is capped by a banded aplite-pegmatite (AP).The geochemistry of Tin-Amzi and Ebelekan granites is nearly comparable. Both are peraluminous (A/CNK=1.10–1.29; ASI=1.17–1.31), sodolithic and fluorine rich with high SiO₂, Al₂O₃, Na₂O+K₂O, Rb, Ga, Li, Ta, Nb, Sn and low FeO, MgO, TiO₂, Ba, Sr, Y, Zr and REE contents. These rare metal Ta bearing granites belong to the P-poor subclass, relating to their P₂O₅ content ( 0.03–0.15 wt.%). Nevertheless, they are distinguished by their concentration of W, Sn and Ta. The Tin-Amzi granite is W–Ta bearing with high W/Sn ratio whereas the Ebelekan granite is Ta–Sn bearing with insignificant W content.At Tin-Amzi the W–Nb–Ta minerals define a sequence formed by W-columbite tantalite followed by wolframoixiolite and finally wolframite showing the effect of hydrothermal overprinting with an extreme W enrichment of the fluids. At Ebelekan, the Sn–Nb–Ta oxides follow a Mn sequence: manganocolumbite→manganotantalite→wodginite+titanowodginite→cassiterite that represents a trend of primary crystallisation resulting from progressive substitution Fe→Mn and Nb→Ta during the magmatic fractionation.     

Geochemical characteristics and spectrometric prospecting in the muscovite-bearing pegmatites and granites,southeastern Aswan,Egypt

Rare metal mineralization of Sn, Nb-Ta and W is encountered in the Gebel Dihmit area （GDA）, southeastern Aswan, Egypt. The mineralization is related to muscovite granites and their pegmatite derivatives. The pegmatites are divided into three types according to their main mineral assemblages： K-feldspar-muscovite-tourmaline, K-feldspar-albite-muscovite and albite-K-feldspar-lepidolite veins. Petrogenetic studies indicate that Sn and Nb-Ta mineralization extends from the late-magmatic stage to the pegmatite and hydrothermal stages of the （GDA） suite. The albite-K-feldspar-lepidolite granite is composed dominantly of albite, lepidolote, and quartz, with topaz, K-feldspar and amblygonite. The accessory minerals are zircon, monazite, pollucite, columbite-tantalite, microlite and Ta-rich cassiterite. Phenocrysts of quartz, topaz and K-feldspar contain abundant inclusions of albite laths and occasional lepidolite crystals along growth zones （snowball texture）, indicating simultaneous crystallization from a subsolvus, residual magma. The origin of the pegmatites is attributed to extreme differentiation by fractional crystallization of a granitic magma. The economic potential for rare metals was evaluated in the geochemical discrimination diagrams. Accordingly, some of the pegmatites are not only highly differentiated in terms of alkalis, but also the promising targets for small-scale Ta and, to a less extent, Sn. The pegmatites also provide the first example of Fe-Mn and Nb-Ta fractionation in successive generations of granites to cassiterite-bearing pegmatites, which perfectly ex- hibit similar fractionation trends established for primary columbite-tantalite in the corresponding categories of pegmatites. Uranium and Th of magmatic origin are indicated by the presence of thorite and allanite, whereas evidence of hydrothermal mineralization is the alteration of rock- foring minerals such as feldspar and the formation of secondary minerals such as uranophane..     

铜绿山Fe-Cu(Au)矽卡岩矿床花岗伟晶岩及其文象结构的成因:来自钾长石40Ar/39Ar年龄、微量元素和石英中流体包裹体的证据

铜绿山Fe-Cu(Au)矿床是长江中下游铁铜成矿带最重要的矽卡岩型矿床之一,矿床的形成与铜绿山石英闪长岩岩株有关.矿区东南部发育有花岗伟晶岩,其形成时间介于石英闪长岩和矽卡岩之间.花岗伟晶岩主要由钾长石、斜长石和石英组成;由石英和钾长石组成的文象结构非常发育.激光阶段加热40Ar/39Ar定年表明,花岗伟晶岩的侵位时间为136.5±0.7 Ma(2σ),与石英闪长岩的侵位时代和铜绿山矿床的成矿时代完全一致. 铜绿山石英闪长岩与花岗伟晶岩的钾长石具有非常相似的主量元素,平均组成分别为Or₈₁Ab₁₈和Or₇₈Ab₂₁.根据岩相学观察和地球化学分析认为,花岗伟晶岩中的文象结构是在快速冷却体系条件下、钾长石晶体生长边界层的SiO₂和Al₂O₃浓度因生长不平衡发生周期性变化而导致石英和钾长石交替生长形成的.铜绿山石英闪长岩和花岗伟晶岩中钾长石的大离子亲石元素(LILE)含量均较高,但与前者相比,花岗伟晶岩中钾长石的Rb、Pb含量明显增加,Ba、Sr含量显著降低,Li、Cs含量略微降低.大离子亲石元素图解(Rb-Ba、La-Ba、K/Ba-Ba、Rb/Sr-Ba)指示花岗伟晶岩是铜绿山石英闪长岩岩浆晚期高度结晶分异演化的结果.但花岗伟晶岩钾长石中Pb、Li、Ga等元素的变化却与岩浆结晶分异演化趋势相悖,表明流体作用在花岗伟晶岩的形成过程中扮演了重要角色.花岗伟晶岩中的石英发育大量熔融包裹体和高盐度流体包裹体,后者的均一温度为260~435 ℃,进一步证实花岗伟晶岩是从流体-熔体共存体系中结晶的.      

Leaching of quartz from precambrian hypabyssal rhyolite porphyry,Llano County,Texas

A Precambrian hypabyssal rhyolite porphyry in central Texas has retained its original texture, although alkali feldspar phenocrysts have inverted and unmixed from zoned, single-phase high-temperature feldspar to zoned microperthite with an intermediate microcline host.Blue quartz phenocrysts owe their color to dispersion by zircon inclusions. Neutron activation analysis reveals substitution of tetravalent cations for silicon in the quartz.In two outcrops, quartz and albite have been completely leached from the rock and K-feldspar added, leaving the texture intact. The leaching was most likely accomplished by residual brine from an overlying Lower Cretaceous evaporite deposit subsequently removed by erosion.