http://www2.ess.ucla.edu/~yin/05-Publications/papers/091-Yin-2006-ESR.pdf

Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation

Abstract In NE China, voluminous granitoids were emplaced in late Paleozoic and Mesozoic times. We report here Sr–Nd–O isotopic and elemental abundance data for two highly evolved granitic plutons, Woduhe and Baerzhe, from the Great Xing'an Mountains. They show a rather “juvenile” Sr–Nd isotopic signature and a spectacular tetrad effect in their REE distribution patterns as well as non-CHARAC (charge-and-radius-controlled) trace element behavior. The emplacement ages are constrained at 130±4 Ma for the Woduhe and 122±5 Ma for the Baerzhe granites by Rb–Sr and Sm–Nd isotope analyses. Both granites are also characterized by low but imprecise initial 87Sr/86Sr ratios of about 0.703. The Nd–Sr isotope data argue for their generation by melting of dominantly juvenile mantle component with subordinate recycled ancient crust. This is largely compatible with the general scenario for much of the Phanerozoic granitoids emplaced in the Central Asian Orogenic Belt. The parental magmas for both the Woduhe and Baerzhe granites have undergone extensive magmatic differentiation, during which intense interaction of the residual melts with aqueous hydrothermal fluids (probably rich in F and Cl) resulted in the non-CHARAC trace element behavior and the tetrad effect of REE distribution. Both the Woduhe and Baerzhe granites show the characteristic trace element patterns of rare-metal granites, but their absolute abundances differ by as much as two orders of magnitude. The oxygen isotope compositions of the two granites have been severely disturbed. Significant 18O depletion in feldspar, but not so much in quartz, suggests that the hydrothermal alteration took place in a temperature condition of 300–500 °C. This subsolidus hydrothermal alteration is decoupled from the late-stage magma–fluid interaction at higher temperatures. Despite the two distinct and intense events of “water–rock” interaction, the Rb–Sr and Sm–Nd geochronological systems seem to have maintained closed, hence, suggesting that the two events occurred shortly after the plutonic emplacements.

Highly evolved juvenile granites with tetrad ree patterns: the woduhe and baerzhe granites from the great xing'an mountains in ne china

Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture, structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO2 content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation, mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, ﬂow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated (including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sign indicating compositional maturity of the continental crust, and they are also closely related to the rare-elemental (metal) mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, REEs, etc.

Highly fractionated granites: Recognition and research

The significant relative mass difference (c. 16%) between the two stable isotopes of Li (approximately 6Li 7.5%, 7Li 92.5%), coupled with broad elemental dispersion in Earth and planetary materials, makes this a system of considerable interest in fingerprinting geochemical processes, determining mass balances, and in thermometry. Natural mass fractionation in this system is responsible for c. 6% variation among materials examined to date (Fig. 1⇓). Although the “modern era” of Li isotope quantification has begun, there are still many questions about the Li isotopic compositions of fundamental materials and the nature of fractionation by important mechanisms that are unanswered (e.g., Hoefs 1997).



 Figure 1. 
Summary of lithium isotopic compositions of Earth and planetary materials. Filled bars are solid samples, open bars are liquids. See text for references and details.



The purpose of this chapter is to summarize the current understanding of Li isotopes in geo- and cosmochemical systems and to indicate (1) where Li isotopes have a high probability of adding new understanding of these systems; (2) where some of the more significant deficits in knowledge exist. The small but burgeoning Li isotope community has not yet compiled the volume of peer-reviewed literature needed to adequately assess even that which has been studied to date. As a result, significant portions of this chapter are based on data reported in abstracts, and as such are more than normally subject to revisions over time. This chapter is anticipated to serve as a starting point for those interested in research incorporating Li isotope geochemistry, or in understanding the state of extant research.

### Experiments in Li isotope fractionation

Knowledge of significant Li isotopic fractionation during basic chemical processes is long standing. The early experiments by Taylor and Urey (1938), in which Li isotopes were fractionated by incomplete extraction of an aqueous solution from a zeolite exchange …

Developments in the Understanding and Application of Lithium Isotopes in the Earth and Planetary Sciences

Iron and lithium isotope systematics of the Hekla volcano, Iceland — Evidence for Fe isotope fractionation during magma differentiation

To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ 7 Li varying from –3.1 to +6.6. The δ 7 Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from –3.1 to +2.5 and overlap with post-Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ 7 Li and elements indicative of degrees of granite differentiation (SiO2, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to res ect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may res ect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100–1100 ppm), quartz (30–140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ 7 Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ 7 Li showing an inverse correlation with Li concentration. This correlation res ects the mixing of isotopically heavy Li in quartz and lighter Li in s uid inclusions, as documented by s uid inclusion compositions (δ

/pdf/lithium-isotopic-systematics-of-granites-and-pegmatites-from-4yt97tl3hc.pdf

Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Granite pegmatite sheets in the continental crust are characterized by very large crystals. There has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids. With this shift there has been a renewed debate about the role of H2O in the petrogenesis of pegmatites. Based on data on nucleation of minerals and new viscosity models for hydrous granite melts, it is argued that H2O is the essential component in the petrogenesis of granite pegmatites. H2O is key to reducing the viscosity of granite melts, which enhances their transport within the crust. It also dramatically reduces the glass transition temperature, which permits crystallization of melts at hundreds of degrees below the thermodynamic solidus, which has been demonstrated by fluid inclusion studies and other geothermometers. Published experimental data show that because H2O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate until melt is substantially undercooled. In a rapidly cooling intrusion, nucleation starts at its highly undercooled margins, followed by inward crystal growth towards its slower-cooling, hotter core. Delay in nucleation may be caused by competition for crystallization by several minerals in the near-eutectic melts and by the very different structures of minerals and the highly hydrated melts. Once a mineral nucleates, however, it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral’s liquidus, assuming components necessary for mineral growth are available along the growth path. Granite pegmatites are apparently able to retain H2O during most of their crystallization histories within the confinement of their wall rocks. Pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling, both of which are facilitated by high H2O (±Li, B, F and P) contents in granite pegmatite melts. Without retention of H2O the conditions for pegmatitic textural growth may be difficult to achieve. Loss of H2O due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in rhyolites. In contrast, slow cooling within a large magma chamber promotes continuous exsolution of H2O from crystallizing magma, growth of equant crystals, and final solidification at the thermodynamic solidus. These are the characteristics of normal granites that distinguish them from pegmatites.

The role of H2O in rapid emplacement and crystallization of granite pegmatites: resolving the paradox of large crystals in highly undercooled melts

We propose that the internally zoned, Li-bearing Tin Mountain pegmatite in the Harney Peak granite-pegmatite system of the Black Hills, South Dakota, crystallized from fluid-rich, compositionally complex melts at ∼400–350 °C. The low crystallization temperatures resulted from the combined fluxing effects of Li, B, P, H 2 O, and carbonate anions. The presence of hydrous silicate melts at temperatures of ∼350 °C is revealed by microthermometric data on primary fluid inclusions cogenetic with crystallized-melt inclusions. Mineral assemblages of the crystallized-melt inclusions and chemistry of bulk-fluid leachates indicate that the melts and fluids contained significant amounts of Li and Na as carbonates and/or borates that acted as powerful fluxes. The low temperatures give a new perspective on the stability of silicate melts in the crust.

Crustal melts below 400 °C

Crystallization conditions and evolution of magmatic fluids in the Harney Peak Granite and associated pegmatites, Black Hills, South Dakota—Evidence from fluid inclusions

http://web.missouri.edu/~lium/pdfs/Papers/Nabelek01tectono.pdf

Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota

Monaliza Catalina Sirbescu

Papers

Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

The role of H2O in rapid emplacement and crystallization of granite pegmatites: resolving the paradox of large crystals in highly undercooled melts

Crustal melts below 400 °C

Crystallization conditions and evolution of magmatic fluids in the Harney Peak Granite and associated pegmatites, Black Hills, South Dakota—Evidence from fluid inclusions

Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota