calamity peak layered granite pegmatite complex black hills south dakota part i structure and emplacement the calamity peak pluton is a satellitic granite pegmatite intrusion located 2 km southwest of the main harney peak granite dome in the black hills south dakota many aspects of the structural development of the calamity peak pluton 1 km 2 mimic those of the larger harney peak dome 100 km 2 thus the model described here for the calamity peak pluton provides a general basis for evaluating the evolution of proterozoic granite pegmatite intrusives of the harney peak granite system the overall structure of the intrusive rocks at calamity peak is that of a dome as shown by the general geometry of the pluton contacts and by a domical pattern of internal textural layering textural layering is defined by alternating bands of pegmatite and fine grained granite 0 1 2 0 m thick fine grained layers are in turn banded on a scale of 2 20 mm owing to modal variations of tourmaline quartz and feldspars textural differentiation of at least some of these layers resulted from segregation of initially homogeneous magmas into pegmatitic upper and aplitic lower components during crystallization relationships between the pluton and regionally dipping schistose country rocks however do not support a simple domal structure schistosity conforms to the contact only along the southern margin of the pluton the side facing with the regional dip country rock schistosity is at a high angle to the contact at the apparent floor of the pluton which is exposed in the core of the dome and along the northern margin the side that opposes the regional dip in order to explain the structural observations it is inferred that the initial intrusive body at calamity peak consisted of a flat lying discordant sheet intruded into steeply dipping quartz mica schist country rock however an arcuate sheet concave downward cannot be ruled out new magma pulses possibly several thousands were successively emplaced along the hanging wall contact of earlier pulses as indicated by the upward truncation of early layers creating a composite layered pluton intrusions of individual magma pulses were localized in three major composite sheets which form structurally and chemically discrete portions of the pluton growth of the pluton proceeded by forcible distension and plastic deformation of overlying country rocks schistosity was molded into a conformable attitude in portions of the hanging wall and pre intrusive country rock lineations were deflected around the pluton the layered granite pegmatite series is crosscut by four successive stages of fracture filling bodies the orientations and compositions of these bodies record the late stages in the structural and fluid evolution of the plutonic system

origin of rhythmic layering in the calamity peak satellite pluton of the harney peak granite south dakota the role of boron abstract the calamity peak satellite granite pluton on the flanks of the harney peak granite south dakota displays prominent mineral and textural layering the latter defined by alternating fine grained and pegmatitic granite superimposed on the fine grained granite layers is a fine scale 0 1 5 cm rhythmic layering defined by modal differences in tourmaline microcline albite and muscovite there is very little variation in the major element mineral compositions in the whole complex analyses by prompt gamma neutron activation gave a b2o3 content of 0 07 in the light layers albite quartz and muscovite and 0 65 in the dark layers tourmaline microcline quartz and albite these contents are essentially proportional to tourmaline abundance in each layer the textural and mineralogic layering is the result of the interplay of tourmaline crystallization and shifts in phase equilibria due to changes in b concentration in the melt initial cotectic crystallization of albite quartz and muscovite increased the activity of fe and mg in the adjacent melt to levels sufficient for tourmaline to form tourmaline crystallization depleted b and decreased the solubility of h2o in the melt causing exsolution of h2o from the melt and further removal of b due to high fluid melt partition coefficient this process also resulted in the precipitation of k feldspar partly at the expense of muscovite and partly due to a slight increase in the solidus temperature once the crystallization front moved to an fe and mg poor region of the melt tourmaline precipitation ceased and the equilibrium shifted back toward quartz muscovite and albite the pegmatitic layers some of which cut across previously formed rhythmic layers and commonly have tourmaline along their borders appear to have crystallized from melt enriched in the exsolved fluid phase

fractionation trends in mica and tourmaline as indicators of pegmatite internal evolution bob ingersoll pegmatite black hills south dakota abstract pegmatitic micas from the bob ingersoll no 1 dike a large internally zoned rare element pegmatite body have been analyzed for trends of compositional variation that indicate the sequence of crystallization and record the progressive compositional differentiation of the pegmatite the dike consists of six primary zones and one replacement assemblage 2m 1 muscovite occurs in the border and wall zones and outer parts of the first intermediate zone intermediate li micas of mixed polytype but with a dominant 2m 1 component occur in inner parts of the pegmatite including the core and areas of contact between intermediate zones and the core 2m 2 lepidolite occurs as a replacement of k feldspar along margins between the first intermediate zone and core fractionation trends of the micas include increasing li substitution decreasing rb cs and increasing ta nb ta in a general sequence from the outer wall zone to the pegmatite core but with considerable overlap among intermediate zones when compared to coexisting tourmaline overlapping compositional trends of the interior zones suggest overlap in timing of crystallization and interzonal geochemical communication border zone mineral compositions do not fit major variation trends due to the combined effects of wallrock contamination replacement of early microcline by lithian mica during differentiation of the pegmatite and possibly rapid disequilibrium crystallization strong zoning of mica and tourmaline crystals reflects disequilibrium conditions as crystalline pegmatite reacted with compositionally evolving fluids lithian mica and lepidolite that replaced early formed k feldspar may have been generated by fluids that were in equilibrium with a residual silicate melt in the pegmatite core aqueous fluids exsolved from silicate melt appear to have leaked from the pegmatite throughout much of its crystallization history and the late stage li rb and cs enriched fluids contributed to the exomorphic halo surrounding the pegmatite

are strongly peraluminous magmas derived from pelitic sedimentary sources igneous compositions that require the presence of a phase more aluminous than biotite are here designated as strongly peraluminous ps such compositions are common but subordinate in felsic rocks and exceedingly rare in more mafic rocks the term s type has been applied to many igneous rocks based largely or even solely upon their ps compositions neglecting or placing less emphasis on other criteria for sedimentary parentage in chappell and white s i and s type classification system the s type designation was in fact initially restricted to granites which were thought to have been derived from a chemically mature strongly weathered pelitic shaly source application of chappell s and white s criteria together with others based upon isotopic and phase equilibria considerations leads to the following conclusions about ps igneous rocks worldwide 1 very few ps rocks are derived entirely from pelitic sources 2 a majority are derived largely from intermediate to felsic crustal sources including

internal differentiation of rare element pegmatites effects of boron phosphorus and fluorine lithium rich rare element pegmatites are characterized by high concentrations of b p and f the interactions of these components with h2o and rare alkalis lower liquidas and solidus temperatures enhance silicate liquid h2o miscibility and control partitioning and concentration of group i elements and higher field strength cations boron f and perhaps p may form peralkaline na and li species that promote early saturation in mica quartz activities of f p and especially b are largely unbuffered throughout crystallization concentration of these components through fractional crystallization involving muscovite generates a peralkaline na aluminosilicate rich melt or vapor from which albitites rich in tourmaline phosphates f rich micas beryl zircon and nb ta sn oxides crystallize phase equilibrium experiments with peraluminous b p f rich rhyolite obsidian macusanite simulate many features of rare element pegmatites especially at h2o undersaturated conditions

the generation and crystallization conditions of the proterozoic harney peak leucogranite black hills south dakota usa petrologic and geochemical constraints the mineralogy petrology and geochemistry of the proterozoic harney peak granite black hills south dakota were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare element pegmatites the granite was emplaced at 3 4 kbar as multiple sills and dikes into quartz mica schists at the culmination of a regional high temperature low pressure metamorphic event principally along the periphery of the main pluton and in satellite intrusions the sills segregated into granite pegmatite couplets the major minerals include quartz k feldspar sodic plagioclase and muscovite biotite mg no molar mgo mgo feo 0 32 0 38 is the predominant ferromagnesian mineral in the granite s core whereas at the periphery of the main pluton and in the satellite intrusions tourmaline mg no 0 18 0 48 is the dominant ferromagnesian phase almandine spessartine garnet is also found in the outer intrusions there is virtually a complete overlap in the wide concentration ranges of sio2 cao mgo feo sr zr w of the biotite and tourmaline bearing granite suites with no discernable differentiation trends on harker diagrams precluding the derivation of one suite from the other by differentiation following emplacement this is consistent with the oxygen isotope compositions which are 11 5 0 6 for the biotite granites and 13 2 0 8 for the tourmaline granites suggesting derivation from different sources the concentrations of tio2 and possibly ba are higher and of mno and b are lower in the biotite granites the normative orthoclase albite ratio is extremely variable ranging from 0 26 to 1 65 in the biotite granites to 0 01 1 75 in the tourmaline granites very few sample compositions fall near the high pressure watersaturated haplogranite minima eutectic trend indicating that the granites for the most part are not minimum melts generated under conditions with a text h text 2 text o 1 instead most biotite granites are more potassic than the water saturated minima and eutectics and in analogy with experimentally produced granitic melts they are best explained by melting at 6 kbar a text h text 2 text o 1 and temperatures 800 c such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals nabelek et al 1992 several of the tourmaline granite samples contain virtually no k feldspar and have oxygen isotope equilibration temperatures 716 775 c therefore they must represent high temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water saturated haplogranite minima or during fractionation of intruded melts into granite pegmatite couplets accompanied by volatile aided differentiation of the alkali elements the indicated high temperatures a text h text 2 text o 1 the relatively high tio2 and ba concentrations and the relatively low partial 18 text o values of the biotite granites suggest that they were generated by high extent biotite dehydration melting of an immature archean metasedimentary source the ascent of the hot melts may have triggered low extent muscovite dehydration melting of schists higher in the crust producing the high b low ti melts comprising the periphery of the main pluton and the satellite intrusions alternatively the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse with the low partial 18 text o biotite granites generated at a deeper hotter region and the high partial 18 text o tourmaline granites at a higher cooler region of the crust the low ti and high b concentrations in the high partial 18 text o melts resulted in the crystallization of tourmaline rather than biotite which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions

archean rocks of the black hills south dakota reworked basement from the southern extension of the trans hudson orogen the little elk and the bear mountain terranes on the margin of the precambrian core of the black hills provide the only known exposures of archean rocks between the archean wyoming and superior provinces the little elk terrane consists of a supracrustal sequence dominated by biotite feldspar gneiss meta arkose or metatuff that is intruded by the slightly metaluminous to peraluminous calc alkaline little elk granite which has a u pb intercept age of 2 549 11 m y a biotite plagioclase schist metagraywacke dominates the bear mountain terrane and is intruded subparallel to foliation by the peraluminous granite at bear mountain an isochron calculated from six severely discordant u pb zircon analyses yields an age of 2 393 230 m y the archean terranes participated in the trans hudson orogenic event suggested by a published rb sr age for the little elk granite 1 850 m y 1 87 1 97 gaigneous activity in the black hills and the similarity in orientation of the proterozoic deformational fabric with geophysical anomalies interpreted to be associated with the trans hudson orogen the metamorphic conditions estimated for the archean rocks may be the result of a thermal event that produced the proterozoic harney peak granite and thermal high northeast of lead south dakota the unexposed precambrian basement is probably dominated by granites similar to the little elk granite and supracrustal rocks consisting of continental margin deposits based on the ree contents of the archean schists and on the heavy mineral assemblages in the uraniferous nature of and the metasedimentary clasts in the lower proterozoic box elder creek formation the ages and the relatively long lived crustal sources for these archean granites and the similarity of the inferred basement characteristics to those of rocks in the wyoming province suggest that the black hills archean rocks may be the easternmost exposures of this province the probable trans hudsonian reworking of the black hills proterozoic rocks and their similarity to those in northern saskatchewan imply that the black hills occupies a tectonic position comparable to that of rocks described from the cree lake zone of saskatchewan

petrogenetic relationships between pegmatite and granite based on geochemistry of muscovite in pegmatite wall zones black hills south dakota usa abstract the compositions of large samples of granitic pegmatite wall zones have been determined for a suite of ten pegmatites of diverse geochemical character and degree of compositional evolution in the keystone area of the black hills whole rock compositions are strongly peraluminous and they deviate substantially from the granite minimum composition in quartz albite orthoclase normalized components showing considerably more scatter than harney peak granite whole rocks wall zone minerals are commonly coarsely segregated leading to large modal variability among whole rocks these features make whole rock samples of wall zones unsuitable for the determination of initial pegmatite bulk compositions trace and minor element compositions of muscovite separates from the wall zones were thus determined to eliminate the effects of modal variability on trace element concentrations so that geochemical differences between pegmatites could be modeled estimates of initial pegmatite melt trace element concentrations range from 800 4000 ppm rb 100 1000 ppm cs 200 2000 ppm li and 1 50 ppm ba trace element concentrations of muscovite from a given pegmatite generally cluster together although several show considerable intra pegmatite scatter and there are large overlaps among different pegmatites the geochemical characteristics of samples from the etta pegmatite indicate mixing with and assimilation of country rocks exceptionally low rb cs ratios of muscovite from the etta pegmatite are similar to those of muscovite from k feldspar rich assemblages of other pegmatites where the rb concentration of melt may have been buffered by crystallizing assemblages that had bulk rb distribution coefficients close to 1 the large degree of scatter of geochemical parameters among these pegmatites precludes derivation of the entire suite by either single stage crystallization or partial melting processes nor can the pegmatites be satisfactorily related by any fractionation trajectory from a single starting composition fractional crystallization is difficult to model due to an apparent geochemical decoupling that exists among several of these elements in different pegmatites e g rb from cs or li from rb cs and nonsystematic variation of ba concentrations however the magnitude of observed rare element enrichments could be achieved by 75 90 fractional crystallization of melts that crystallized harney peak granite in nearby exposures the inter pegmatite scatter and apparent geochemical decoupling that characterize the suite indicate an origin by partial melting of a heterogeneous source rock or rock sequence followed by an intermediate episode of fractional crystallization prior to final emplacement of the pegmatite melts additional compositional variations among samples from the wall zones of individual pegmatites were caused by processes during or after emplacement including local interaction with host metamorphic rocks and fractionation during internal evolution of the pegmatites as they crystallized

inter and intra crystal ree variations in apatite from the bob ingersoll pegmatite black hills south dakota abstract concentrations of rare earth elements ree have been measured on a suite of apatite crystals from an internally zoned granitic pegmatite enriched in li b be f nb ta sn and u with a cameca ims 3f ion microprobe using energy filtering an apatite specimen from the tin mountain pegmatite analyzed previously by isotope dilution was used as a standard the chondrite normalized pattern determined with the ion microprobe closely matches the pattern determined by isotope dilution with maxima at sm and dy and minima at nd and er apatite samples from the bob ingersoll pegmatite show a large range of ree patterns and concentrations in one case apatite crystals within millimeters show differences in ree concentrations and pattern shapes including a switch from positive to negative eu anomalies samples from several mineral assemblages show patchy individual crystal zoning with respect to the ree that is not mirrored by major element zoning this indicates disequilibrium conditions on a small scale consistent with rapid growth from a melt fluid system that was either of locally heterogeneous melt structure or in which the melt structure fluctuated rapidly as volatile rich minerals such as tourmaline crystallized these effects may be coupled with non ideal partitioning of ree in a heterogeneous mixture of melt aqueous fluid and crystals ree concentrations in apatite samples from the different pegmatite zones indicate a large variation in outer zones 10 500x chondrite high concentrations 100 1000x chondrite near the pegmatite core and very low concentration in the core 2 20x chondrite patterns are flat to slightly inclined ce yb 1 to 5 and most samples have positive eu anomalies the magnitude of positive eu anomalies decreases with inward position in the pegmatite possibly indicating a progressive increase in o 2 and a sharp increase may be indicated by systematic ce depletion in apatite from the pegmatite core ree specific volatile complexes may contribute to variations including unusual kinks observed in ree patterns of apatite from mineral assemblages in upper parts of the pegmatite

trace element constraints on pegmatite genesis tin mountain pegmatite black hills south dakota the tin mountain pegmatite is a small zoned granitic body that is extremely enriched in rb and li but has moderate concentrations of sr and ba these trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less evolved granitic melt to have produced the parental liquid to the tin mountain pegmatite batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in rb and li to be the parental liquid or that had sr and ba concentrations that were much higher than those estimated for the parental liquid the modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated harney peak granite gives calculated melt compositions with either lower sr and ba concentrations or inadequate rb and li enrichments to be the parent liquid of the pegmatite at least two variants from simple crystal liquid fractionation models can however successfully account for the derivation of the parent liquid 1 generation of a rb li ba and sr rich granitic melt outside of the compositional range of the sampled portions of the harney peak granite complex by low degrees of partial melting of metasedimentary rocks found in the black hills followed by moderate extents of fractional or equilibrium crystallization 2 derivation from harney peak granite via a complex multi stage crystal liquid fractionation process such as progressive equilibrium crystallization

combined escape tectonics and subduction rollback back arc extension a model for the evolution of tertiary rift basins in thailand malaysia and laos the tertiary rift basins of thailand and adjacent countries show considerable variability in the timing of rift initiation termination the timing and magnitude of thermal subsidence and the timing and intensity of inversion episodes the rift basins developed on continental blocks that were extruded southeastwards hence their development must be tied into himalayan extrusion tectonics current tectonic models propose that the tertiary basins opened up as pull apart basins associated with strike slip faults published geochronology of strike slip fault zone rocks mapping of fault patterns in the tertiary basins and mapped releasing restraining bend geometries all indicate that in thailand major sinistral strike slip motion ceased at about 30 ma prior to the formation of most rift basins the thailand the effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during nw se to ne sw himalayan compression dextral slip was not responsible for opening most of the rift basins in thailand an alternative mechanism to open the rift basins is subduction rollback of the indian plate to the west of thailand it is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non uniform lithospheric extension deep sag basins and the diachronous onset and termination of rifting

radiogenic 87sr its mobility and the interpretation of rbsr fractionation trends in rare element granitic pegmatites abstract the extent of fractionation of rb and sr is routinely used in petrogenetic modelling of igneous processes including internal fractionation of individual pegmatites as well as large scale evolution of pegmatite groups and fields however highly evolved granitic pegmatites may contain as much as 14000 ppm rb and less than 150 ppm sr the total sr in k feldspar and micas from geologically old and rb rich pegmatites may consist predominantly of radiogenic 87 sr which obscures the original relationship of rb to common sr at the time of crystallization a subtraction of radiogenic 87 sr calculated from the rb content and age of emplacement is possible but it commonly results in negative concentrations of sr the relative immobility of rb analytically determined isotopic composition of sr apparent ages of the rb sr bearing minerals high concentration of 87 sr in coexisting rb poor phases and experimental evidence indicate that post crystallization migration of radiogenic 87 sr is significant where isotopic data are not available rb sr trends in geologically old and highly fractionated pegmatites are misleading and cannot be used for geochemical interpretation of pegmatite derivation or evolution

pegmatite wallrock interactions black hills south dakota progressive boron metasomatism adjacent to the tip top pegmatite abstract interaction between country rock and fluids derived from the tip top pegmatite has resulted in a series of boron enriched assemblages between unaltered quartz mica schist to the pegmatite contact is a succession of four mineral assemblages 1 1 quartz biotite potassium feldspar assemblage q b k which consists essentially of the original metamorphic silicate assemblage plus anomalously high amounts of modal tourmaline 2 2 quartz biotite tourmaline assemblage q b t 3 3 tourmaline quartz muscovite assemblage t q m 4 4 tourmaline quartz assemblage t q alkali elements cs rb k li sio 2 and ba show a decrease from the q b k assemblage to the t q assemblage a1 2 o 3 ga b total fe and zn increase moderately from the q b k assemblage to the t q assemblage the mineral chemistries also change considerably the mg mg fe 2 ratios in biotites range from 0 54 to 0 50 in samples from the q b k assemblage to 0 39 in the q b t assemblage the range in tourmaline end member components from the q b k assemblage to the t q assemblage is as follows q b k dravite 63 schorl 23 elbaite 05 buergerite 09 t q dravite 23 schorl 37 elbaite 17 buergerite 23 observed variations in mineral assemblage and whole rock chemistry within the alteration zone appear to a first approximation to be a function of b 2 o 3 boron metasomatism and k 2 o alkali leaching the breakdown of feldspar and biotite may be approximated by reactions 2hcl 2 k na alsi 3 o 8 ai 2 k na cl al 2 sio 5 5sio 2 h 2 o and 2 annite sio 2 5al 2 sio 5 2nacl 6h 3 bo 3 ai 2 tourmaline 2kcl 7h 2 o the alteration zone may represent either a single episode b cs li rb enriched fluid or multiple episodes b zn mn fluid and cs li rb fluid of pegmatite fluid schist interactions in both situations b in the aqueous fluid from the pegmatite reacts with the schist breaking down sheet silicate traps for cs rb li and k and forming tourmaline rich assemblages

an investigation on slowness weighted ccp stacking and its application to receiver function imaging common conversion point ccp stacking of receiver functions is a widely used technique to image velocity discontinuities in the mantle the ccp imaging technique assumes that receiver functions are composed solely of p to s conversions at velocity boundaries whose depths can be mapped out through their arrival times the multiple reflections at shallow boundaries with large velocity contrasts such as the base of unconsolidated sediments and the moho can lead to artificial structures in the ccp images we develop a refined ccp stacking method that uses relative slowness as a weighting factor to suppress the multiples slowness weighted ccp stacking swccp we conduct extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images we apply this technique to receiver function data of necessarray in china and the transportable array in western us and find most of the events in the depth range of 200 400 km shown in the regular ccp images are eliminated the swccp images on the other hand reveal a clear negative event under some parts of the two arrays indicating the presence of low velocity layer above the 410 km discontinuity which was reported by previous studies

vapor undersaturated experiments with macusani glass h 2 o at 200 mpa and the internal differentiation of granitic pegmatites vapor undersaturated fractional crystallization experiments with macusani glass macusanite a peraluminous rhyolite obsidian at 200 mpa yield mineralogical fabrics and zonation and melt fractionation trends that closely resemble those found in zoned granitic pegmatites and other granitoids of comparable composition typically peraluminous li be ta rich deposits the zonation from the edge of charges inward is characterized by 1 fine grained sodic feldspar quartz border zones 2 a fringe of very coarse grained graphic quartz feldspar intergrowths that flair radially toward melt and terminate with nearly monophase k feldspar 3 cores of very coarse grained nearly monominerallic quartz or virgilite lialsi5o12 mica and 4 late stage fine grained albite mica intergrowths that are deposited from alkaline na rich interstitial melt at vapor saturation similar experimental products have been observed in compositionally simpler less evolved systems liquid lines of descent from initially h2o undersaturated runs are marked by a decrease in sio2 and increases in na k b p f h2o and a variety of trace lithophile cations these trends are believed to be governed by three factors 1 disequilibrium growth of feldspars quartz via metastable supersaturation 2 fractionation of melt toward sio2 depleted na rich compositions due to increases in b p and f and 3 changes in nucleation and growth rates mostly as a function of the h2o content of melt x w m in contrast experiments that are cooled below the liquidus from the field of melt aqueous vapor london et al 1988 fail to replicate pegmatitic characteristics in most respects on the basis of these and other experiments we suggest that the formation of pegmatite fabrics stems primarily from fractional crystallization in volatile rich melts and that enrichments in normally trace lithophile elements result from melt differentiation trends toward increasingly alkaline silica depleted compositions although vapor saturation at near solidus and subsolidus conditions may promote extensive recrystallization an aqueous vapor phase does not appear to be necessary for the generation of most of the salient characteristics of pegmatites