Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex, Western Mongolia

Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO2) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H2O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K2O/Na2O values and LILE concentrations with increasing SiO2 as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K2O/Na2O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt

Petrogenesis of Pyroxenites and Melt Infiltrations in the Ultramafic Complex of Beni Bousera, Northern Morocco

The origin of pyroxenites and their relation to melt migration in the mantle have been investigated in two pyroxenite-rich zones in the Beni Bousera massif. Based on combined field, microtextural, mineralogical and geochemical observations, the pyroxenites were separated into four types. Type-I Cr-diopside websterites contain bright green diopside and have primitive bulk Ni, Cr and Mg-number. Their trace element systematics are characterized by slight light rare earth element (LREE) enrichment compared with the middle (MREE) and heavy (H)REE, and negative high field strength element (HFSE) anomalies in bulk-rock and mineral compositions suggesting that they result from melting of metasomatized mantle. Trace element concentrations of melts calculated to be in equilibrium with Type-I cpx have a subduction-like signature and show a close similarity to certain lavas erupted in the Alboran Basin. Calculated mineral equilibration temperatures of ∼1200 to 1350°C are close to the basalt liquidus and higher than for other pyroxenite types in Beni Bousera, which generally yield 45 to 20-30 kbar. Type-III pyroxenites display strong variations of LREE and HFSE depletion and strong bulk Nb/Ta fractionation. Calculated melts in equilibrium with augitic cpx are variably enriched in incompatible trace elements similar to intraplate basalts. Type-IV pyroxenites are composed of green diopside, opx, garnet and plagioclase and/or spinel. Whole-rocks have high Na2O, CaO and Al2O3 concentrations and high Mg-number, are HREE depleted, and have positive Eu and Sr anomalies. Garnets are characterized by low HREE/MREE and positive Eu anomalies. The absence of bulk-rock HREE enrichment indicates a metamorphic origin for this garnet, which is corroborated by the presence of Al-rich metamorphic spinels. Relict magmatic plagioclase indicates a shallower (<10 kbar) crustal origin for these pyroxenites. Their metamorphic assemblage yields temperatures and pressures of 800-980°C and 14 kbar, indicating a pressure increase during the metamorphic overprint. The whole-rock geochemistry of Type-IV pyroxenites is comparable with that of rocks from the lower crustal section of the Kohistan (northern Pakistan) paleo-arc, indicating a possible origin of these rocks as cumulates in the deeper arc crust and subsequent delamination into the underlying mantl

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

We present the geochemistry and intrusion pressures of granitoids from the Kohistan batholith, which represents, together with the intruded volcanic and sedimentary units, the middle and upper arc crust of the Kohistan paleo-island arc. Based on Al-in-hornblende barometry, the batholith records intrusion pressures from ~0.2 GPa in the north (where the volcano-sedimentary cover is intruded) to max. ~0.9 GPa in the southeast. The Al-in-hornblende barometry demonstrates that the Kohistan batholith represents a complete cross section across an arc batholith, reaching from the top at ~8–9 km depth (north) to its bottom at 25–35 km (south-central to southeast). Despite the complete outcropping and accessibility of the entire batholith, there is no observable compositional stratification across the batholith. The geochemical characteristics of the granitoids define three groups. Group 1 is characterized by strongly enriched incompatible elements and unfractionated middle rare earth elements (MREE)/heavy rare earth element patterns (HREE); Group 2 has enriched incompatible element concentrations similar to Group 1 but strongly fractionated MREE/HREE. Group 3 is characterized by only a limited incompatible element enrichment and unfractionated MREE/HREE. The origin of the different groups can be modeled through a relatively hydrous (Group 1 and 2) and of a less hydrous (Group 3) fractional crystallization line from a primitive basaltic parent at different pressures. Appropriate mafic/ultramafic cumulates that explain the chemical characteristics of each group are preserved at the base of the arc. The Kohistan batholith strengthens the conclusion that hydrous fractionation is the most important mechanism to form volumetrically significant amounts of granitoids in arcs. The Kohistan Group 2 granitoids have essentially identical trace element characteristics as Archean tonalite–trondhjemite–granodiorite (TTG) suites. Based on these observations, it is most likely that similar to the Group 2 rocks in the Kohistan arc, TTG gneisses were to a large part formed by hydrous high-pressure differentiation of primitive arc magmas in subduction zones.National Science Foundation (U.S.) (Grant EAR 6920005

Petrology and Mineral Chemistry of Lower Crustal Intrusions: the Chilas Complex, Kohistan (NW Pakistan)

Mineral major and trace element data are presented for the main rock units of the Chilas Complex, a series of lower crustal intrusions emplaced during initial rifting within the Mesozoic Kohistan (paleo)-island arc (NW Pakistan). Detailed field observations and petrological analysis, together with geochemical data, indicate that the two principal units, ultramafic rocks and gabbronorite sequences, originate from a common parental magma, but evolved along different mineral fractionation trends. Phase petrology and mineral trace element data indicate that the fractionation sequence of the ultramafic rocks is dominated by the crystallization of olivine and clinopyroxene prior to plagioclase, whereas plagioclase precedes clinopyroxene in the gabbronorites. Clinopyroxene in the ultramafic rocks (with Mg-number [Mg/(Fetot + Mg] up to 0·95) displays increasing Al2O3 with decreasing Mg-number. The light rare earth element depleted trace element pattern (CeN/GdN ∼0·5-0·3) of primitive clinopyroxenes displays no Eu anomaly. In contrast, clinopyroxenes from the gabbronorites contain plagioclase inclusions, and the trace element pattern shows pronounced negative anomalies for Sr, Pb and Eu. Trace element modeling indicates that in situ crystallization may account for major and trace element variations in the gabbronorite sequence, whereas the olivine-dominated ultramafic rocks show covariations between olivine Mg-number and Ni and Mn contents, pointing to the importance of crystal fractionation during their formation. A modeled parental liquid for the Chilas Complex is explained in terms of mantle- and slab-derived components, where the latter component accounts for 99% of the highly incompatible elements and between 30 and 80% of the middle rare earth elements. The geochemical characteristics of this component are similar to those of a low percentage melt or supercritical liquid derived from subducted mafic crust. However, elevated Pb/Ce ratios are best explained by additional involvement of hydrous fluids. In accordance with the crystallization sequence, the subsolidus metamorphic reactions indicate pressures of 0·5-0·7 GPa. Our data support a model of combined flux and decompression melting in the back-ar

Construction of the granitoid crust of an island arc part I: geochronological and geochemical constraints from the plutonic Kohistan (NW Pakistan)

We present major and trace element analyses and U-Pb zircon intrusion ages from I-type granitoids sampled along a crustal transect in the vicinity of the Chilas gabbronorite of the Kohistan paleo-arc. The aim is to investigate the roles of fractional crystallization of mantle-derived melts and partial melting of lower crustal amphibolites to produce the magmatic upper crust of an island arc. The analyzed samples span a wide calc-alkaline compositional range (diorite-tonalite-granodiorite-granite) and have typical subduction-related trace element signatures. Their intrusion ages (75.1±4.5-42.1±4.4Ma) are younger than the Chilas Complex (~85Ma). The new results indicate, in conjunction with literature data, that granitoid formation in the Kohistan arc was a continuous rather than punctuated process. Field observations and the presence of inherited zircons indicate the importance of assimilation processes. Field relations, petrographic observations and major and trace element compositions of the granitoid indicate the importance of amphibole fractionation for their origin. It is concluded that granitoids in the Kohistan arc are derivative products of mantle derived melts that evolved through amphibole-dominated fractionation and intra crustal assimilatio

Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex, Western Mongolia

Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO2) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H2O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K2O/Na2O values and LILE concentrations with increasing SiO2 as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K2O/Na2O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt

Oxygen Isotope Trajectories of Crystallizing Melts: Insights from Modeling and the Plutonic Record

Elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth. The δ^(18)O value of a melt, however, can also increase through closed-system fractional crystallization. In order to quantify the change in melt δ^(18)O due to crystallization, we develop a detailed closed-system fractional crystallization mass balance model and apply it to six experimentally- and naturally-determined liquid lines of descent (LLDs), which cover nearly complete crystallization intervals (melt fractions of 1 to <0.1). The studied LLDs vary from anhydrous tholeiitic basalts to hydrous high-K and calc-alkaline basalts and are characterized by distinct melt temperature-SiO_2 trajectories, as well as, crystallizing phase relationships. Our model results demonstrate that melt fraction-temperature-SiO_2 relationships of crystallizing melts, which are strongly a function of magmatic water content, will control the specific δ^(18)O path of a crystallizing melt. Hydrous melts, typical of subduction zones, undergo larger increases in δ^(18)O during early stages of crystallization due to their lower magmatic temperatures, greater initial increases in SiO_2 content, and high temperature stability of low δ^(18)O phases, such as oxides, amphibole, and anorthitic plagioclase (versus albite). Conversely, relatively dry, tholeiitic melts only experience significant increases in δ^(18)O at degrees of crystallization greater than 80%. Total calculated increases in melt δ^(18)O of 1.0 to 1.5‰ can be attributed to crystallization from ∼50 to 70 wt.% SiO_2 for modeled closed-system crystallizing melt compositions. As an example application, we compare our closed system model results to oxygen isotope mineral data from two natural plutonic sequences, a relatively dry, tholeiitic sequence from the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex (South Africa) and a high-K, hydrous sequence from the arc-related Dariv Igneous Complex (Mongolia). These two sequences were chosen as their major and trace element compositions appear to have been predominantly controlled by closed-system fractional crystallization and their LLDs have been modeled in detail. We calculated equilibrium melt δ^(18)O values using the measured mineral δ^(18)O values and calculated mineral-melt fractionation factors. Increases of 2-3‰ and 1-1.5‰ in the equilibrium melts are observed for the Dariv Igneous Complex and the UUMZ of the Bushveld Complex, respectively. Closed-system fractional crystallization model results reproduce the 1‰ increase observed in the equilibrium melt δ^(18)O for the Bushveld UUMZ, whereas for the Dariv Igneous Complex assimilation of high δ^(18)O material is necessary to account for the increase in melt δ^(18)O values. Assimilation of evolved supracrustal material is also confirmed with Sr and Nd isotope analyses of clinopyroxene from the sequence. Beginning with a range of mantle-derived basalt δ^(18)O values of 5.7‰ (“pristine” mantle) to ∼7.0‰ (heavily subduction-influenced mantle), our model results demonstrated that high-silica melts (i.e. granites) with δ^(18)O of up to 8.5‰ can be produced through fractional crystallization alone. Lastly, we model the zircon-melt δ^(18)O fractionations of different LLDs, emphasizing their dependence on the specific SiO_2-T relationships of a given crystallizing melt. Wet, relatively cool granitic melts will have larger zircon-melt fractionations, potentially by ∼1.5‰, compared to hot, dry granites. Therefore, it is critical to constrain zircon-melt fractionations specific to a system of interest when using zircon δ^(18)O values to calculate melt δ^(18)O

Introduction to Geology

This course introduces students to the basics of geology. Through a combination of lectures, labs, and field observations, we will address topics ranging from formation of the elements, mineral and rock identification, and geological mapping to plate tectonics, erosion and climate engineering