Degree Of Magmatic Differentiation Research Articles

Geochemistry, Monazite U–Pb Dating, and Li–Nd Isotopes of the Madi Rare Metal Granite in the Northeastern Part of the North China Craton

The Madi rare metal granite is a complex massif, which contains a variety of rare metals, such as Nb, Ta, Li, and Be. In this paper, the geochemical characteristics of the granite were obtained by multi‐collector inductively coupled mass spectrometry (MC‐ICP‐MS). The precise crystalline age of the granite was obtained from monazite U‐Pb dating, and the source of the granite was determined using Li‐Nd isotopes. The Madi rare metal granite is a high‐K (calc‐alkaline), peraluminous, S‐type granite. The U‐Pb monazite age indicates that the crystalline age of the granite is 175.6 Ma, which is Early Jurassic. The granite is characterized by a relatively wide range of δ7Li values (+2.99‰ to +5.83‰) and high lithium concentrations (181 ppm to 1022 ppm). The lithium isotopic composition of the granite does not significantly correlate with the degree of magmatic differentiation. An insignificant amount of lithium isotope fractionation occurred during the granitic differentiation. The lithium isotopic composition of the granite significantly differs from that of the wall rock, but it is very similar to that of a primitive mantle peridotite xenolith (mean δ7Li value +3.5‰). The plot of Li concentration versus δ7Li indicates that the Li isotopic composition of the granite is similar to that of island arc lavas. Based on the above‐described evidence, the granite was mainly derived from the crust, but it was contaminated by a deep granitic magma.

Laurite and Ir-Osmium from Plagioclase Lherzolite of the Yoko–Dovyren Mafic–Ultramafic Pluton, Northern Baikal Region

The near-bottom part of the Yoko-Dovyren layered ultramafic-mafic intrusion host the Baikal deposit of Cu–Ni sulfide ores with Pt–Pd mineralization, whereas horizons and pockets of low sulfide ores with Pt–Pd mineralization occur at higher stratigraphic levels, including the boundary between strata of troctolite and gabbronorite, within these rocks, as well as in strata of peridotite at the lower part of the intrusion. This paper represents a new (for the Yoko-Dovyren intrusion) type of “refractory IPGE-mineralization” discovered in the lower peridotite ranging from two-pyroxene-plagioclase-bearing lherzolite. This mineralization occurs in thin intercalations of plagioclase lherzolite containing as much as 7% of alumochromite, up to 50 ppb Ru, 15 ppb Ir, and 60 ppb Pt. Crystals of cumulate alumochromite with 0.2–0.8 wt % TiO2 contain hexagonal plates of Ir-osmium up to 5 m in size. Crystals of cumulate alumochromite with 1.2–2.8 wt % TiO2 host pentagonal dodecahedrons of laurite up to 4 m in size. One of the alumochromite crystals with an inclusion of Os-poor laurite was found inside a crystal of cumulate olivine Fo86. Intergrowth of laurite and Ir-osmium enclosed in alumochromite with 1.1% TiO2 was observed in one case. Laurite from Yoko-Dovyren contains 93–66%, predominantly 92–82%, RuS2 endmember (n = 10); 3–20, predominantly 5–12%, OsS2 endmember; 4–5% IrS2 endmember; and up to 0.7% Pd and 0.5% Au. Ir-osmium is divided into two groups by composition. The first group is enriched in Os (58–73 wt %, on average 64 wt %) and Ru (3–8 wt %, on average 5 wt %), contains 24–34 wt % Ir (n = 4), up to 1.4 wt % Au, and no Pt. Compositions of the second group have 57–58 wt % Os, 27–30 wt % Ir, 1.5–5.5 wt % Ru, approximately 10 wt % Pt (n = 3), and up to 0.2 wt % Pd. The Cr# and Fe2+/(Fe2+ + Mg) values, which range within 58–69 and 61–72, respectively, are identical in alumochromite with both enclosed laurite and Ir-osmium. Alumochromite, relatively enriched in Ti, crystallized slightly later, suggesting later crystallization for hosted laurite. Occurrence of Ir-osmium seems to indicate a picritic magma undersaturated with sulfide sulfur during bulk crystallization of alumochromite Judging from the diagram from (Brennan and Andrews, 2001), intergrowths of laurite and Ir-osmium, evidence that their probable crystallization temperature did not exceed 1250°C. The presence of own minerals of Ru, Os, Ir in the rocks, containing the first ppb of these PGE shows startling degree of magmatic differentiation. In the matrix of plagioclase lherzolites, containing laurite and Ir-osmium, in association with phlogopite, pargasite, pentlandite, troilite and chalcopyrite there were found the smallest crystals of geversite, sperrilite, insizwaite, niggliite, naldrettite, zvyagintsevite, in association with serpentine and chlorite–native platinum, Pd-platinum, osarsite, irarsite, platarsite.

Fractionation of rare-earth elements during magmatic differentiation and weathering of calc-alkaline granites in southern Myanmar

AbstractGeochemical characteristics and rare-earth element (REE)-bearing minerals of calc-alkaline granites in southern Myanmar were investigated to identify the minerals controlling fractionation between light and heavyREE(LREE and HREE) during magmatic differentiation and weathering. The granites were classified on the basis of the mineral assemblages into two contrasting groups: allanite-(Ce)- and/or titanite-bearing granites; and more HREE-enriched granites characterized by hydrothermal minerals including synchysite(Y), parisite-(Ce), bastnäsite-(Ce), xenotime-(Y), monazite-(Ce), Y-Ca silicate, waimirite-(Y) and fluorite. This suggests that allanite-(Ce) and titanite are not stable in differentiated magma and HREE are eventually preferentially incorporated into the hydrothermal minerals. The occurrence of theREE-bearing minerals is constrained by the degree of magmatic differentiation and the boundary of two contrasting granite groups is indicated by SiO2contents of ∼74 wt.% or Rb/Sr ratios of ∼3–8. Fractionation between LREE and HREE during weathering of the granites is influenced by weathering resistance of theREE-bearing minerals, i.e. allanite-(Ce), titanite, theREEfluorocarbonates and waimirite-(Y) are probably more susceptible to weathering, whereas zircon, monazite-(Ce) and xenotime-(Y) are resistant to weathering. Ion-exchangeableREEin weathered granites tend to be depleted in HREE relative to the whole-rock compositions, suggesting that HREE are more strongly adsorbed on weathering products or that HREE remain in residual minerals.

Mineralogy and geochemistry of leucogranite from Miniki Gol (Chitral), northern Pakistan: implications for petrogenesis and mineralisation

A body of two-mica leucogranite occurs in the Miniki Gol area of Chitral, northern Pakistan. Exposed at a distance of just 400 m from scheelite mineralisation in the area, the leucogranite consists of quartz, albite, orthoclase, muscovite and garnet together with minor amounts of biotite, tourmaline and apatite. Consistent with these mineralogical features, the geochemical details of the Miniki Gol leucogranite (MGL) manifest its peraluminous trait. The average values of Li, Rb, Pb, Be, W, Ta and U are distinctly higher in MGL than those in low-Ca granites. The average W and Ta abundances in the MGL samples are higher also than the Himalayan (Manaslu) leucogranite. Besides, the abundance of Sn in all the MGL samples is markedly higher than both the low-Ca granite and the Himalayan leucogranite. On the other hand, the levels of Zr, Hf, Y, Th, Nb, Sr, Ba, Zn, V and Ni abundance are distinctly lower in MGL than the average low-Ca granite. The MGL geochemical profile also displays very low levels of light rare earth element (LREE) relative to HREE, negative Eu anomalies and good sympathetic relationships between FeO* and MgO, Sn and Ta, Sr and Ba, Zr and Th and Zr and LREE. All these features signify a high degree of magmatic differentiation, i.e. the highly evolved character of MGL and derivation of its parent melt from a pelitic protolith in a collisional environment. The high levels of Pb, W, Sn, Ta and Be in the MGL point to its metallogenic significance. As the enrichment in these elements, especially Sn and Ta, seems to be a diagnostic feature of Sn, W-bearing granites, the MGL could be potential ore-bearing granite. Furthermore, the occurrence and Zr, Ta-richer character of scheelite in the associated metasedimentary rocks and the presence of beryl in the Miniki Gol pegmatite suggest that the investigated leucogranite may have played a key role in the mineralisation of both tungsten and beryllium in the Miniki Gol area.

Major and Trace Element Characteristics of Apatites in Granitoids from Central Kazakhstan: Implications for Petrogenesis and Mineralization

AbstractThis paper presents abundances of major and trace elements of apatites in granitic rocks associated with different types of ore deposits in Central Kazakhstan on the basis of electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. Our results demonstrate that the concentrations and ratios of elements in apatites from different granitoid rocks show distinct features, and are sensitive to magma evolution, petrogenetic and metallogenetic processes. Apatites in the rocks associated with Mo‐W deposits have high content of F and MnO, low content of Cl, which may be indicative of sedimentary sources, while apatites from a Pb‐Zn deposit show relatively high content of Cl and low F content, which possibly suggest a high water content. In these apatites, Sr contents decrease, while Mn and Y contents increase with magma evolution. This relationship reflects that these elements in apatites are related with the degree of magmatic differentiation. Four types of REE patterns in apatites are identified. Type 1 character of highest (La/Yb)N in apatites of Aktogai porphyry Cu‐Mo deposit, Sayak‐I skarn Cu deposit and Akzhal skarn Pb‐Zn depposit is likely produced by the crystallization of heavy REE‐enriched minerals. Type 2 character of upward‐convex light REE in apatite of Aktogai porphyries likely results from La‐enriched mineral crystallization. Type 3 feature of Nd depletion in apatites of East Kounrad and Zhanet deposits both from Mo‐W deposits primarily inherits the character of host‐rock. Type 4 apatites of Aktogai deposit and Akshatau W‐Mo deposit with wide range of REE contents may suggest that apatites crystallize under a wide temperature range. Three types of apatite with distinct redox states are identified based on Eu anomaly. The Aktogai apatite with slight negative Eu anomaly displays the most oxidized state of the magma, and the apatites of other samples at Aktogai, East Kounrad and Akzhal with moderate negative Eu anomaly show moderate oxidizing condition of these rocks, while the remaining apatites with strong En anomaly indicate a moderate reductive state of these rocks.

Granitoid Types Related to Cretaceous Plutonic Au‐Quartz Vein and Cu‐Fe Skarn Deposits, Kitakami Mountains, Japan

Abstract. Early Cretaceous granitic intrusions are associated with Au‐quartz veins and Cu‐Fe skarns in the the Kitakami Mountains, which are underlain by the late Paleozoic of continental margin‐type sedimentary rocks and Mesozoic accretionary complexes. The plutonic rocks are divided into potassic, high‐Sr/Y calc‐alkaline and low‐Sr/Y calc‐alkaline series. All the metallic mineral deposits are spatially associated with small stocks and plugs; they show no consistent association with the larger plutonic bodies. The plutonic rocks generally belong to the magnetite series but less oxidized in the southwestern part of the Kitakami Mountains where Au‐quartz veins occur. The gold deposits are classified into high and low sulfide types.The high sulfide type contains a high volume of sulfide minerals mostly of chalcopyrite, arsenopyrite and pyrrhotite with low bulk Au/Ag ratios. This type occurs almost exclusively in and surrounding the Orikabe pluton, including two most important gold deposits (Oya and Kohoku) of the Kitakami Mountains. The pluton is composed of potassic gabbroids, potassic granitoids of the shoshonite ‐ high‐K calc‐alkaline series (Orikabe type), and less potassic Sasamori‐type granodiorite. All these rocks belong to a moderately oxidized magnetite series. The Orikabe pluton has one of the lowest initial Sr ratio (0.70392) in the Kitakami Mountains, and the Au‐Cu‐dominant ore components of the high sulfide type Au deposits are considered magmatic in origin carried by the juvenile magmas from the upper mantle.The low sulfide type is generally plain quartz vein with a low volume of sulfides and a high bulk Au/Ag ratio. The associated minerals are often scheelite and/or arsenopyrite and pyrrhotite. The ore deposits include historically famed Au‐quartz veins at Shishiori and Ogayu. They are widespread in the southwestern Kitakami Mountains and may be later than the high sulfide type in age, and are hosted most commonly in the sedimentary rocks, which surround small weakly oxidized magnetite‐series plutons of low to intermediate Sr/Y series. These less differentiated intrusions typically include quartz dior‐ite and granodiorite. Some ore components of this type may have derived from the host sedimentary rocks.Among other mineral deposit types in the region, the largest ore deposit is Kamaishi Cu‐Fe skarn (magnetite ores of 58 MT, Fe 50–64 %; Cu 143 KT). It is related to the high‐Sr/Y series Ganidake granodiorite stock, which is a strongly oxidized magnetite‐series body. In contrast, the second largest deposit in the mountains, Akagane deposit, is a similar‐type skarn but associated with an intrusion classified as less oxidized, ilmenite to intermediate series, and that is intermediate in Sr/Y of calc‐alkaline series granodiorite. Degree of magmatic differentiation appears to be not critical factor in the formation of Au‐quartz vein and Cu‐Fe skarn deposits in the region, but is definitely significant for controlling the distribution of the Mo‐mineralization to the east.

Recent volcanic rocks from Jan Mayen: Low‐degree melt fractions of enriched northeast Atlantic mantle

Isotopic and trace element analyses of recent alkaline volcanic rocks from Jan Mayen were used to characterize their mantle source. The samples, ranging in composition from ankaramites to trachytes, are isotopically homogeneous with 11 of 12 samples within the following ranges: 87Sr/86Sr 0.7035–0.7036, 143Nd/144Nd: 0.51285–0.51290, 206Pb/204Pb: 18.54–18.76, 207Pb/204Pb 15.45–15.49, and 208Pb/204Pb: 38.19–38.51. Geophysical data indicate that the Jan Mayen Ridge represents a microcontinent rifted off the east Greenland continental margin at 36 Ma. The isotopic homogeneity and poor correlation between the degree of magmatic differentiation and Sr‐Nd isotopic composition show that the recent Jan Mayen magmas escaped significant contamination by Precambrian or Lower Paleozoic continental crust. A pervasive Tertiary to recent magmatic infrastructure may explain the lack of contamination with old crustal material. The Jan Mayen alkali basalts have similar trace element and isotopic composition to other oceanic plume basalts and are dominated by low‐degree melts from an enriched mantle component prevalent in the NE Atlantic. The Icelandic alkali basalts are derived from similar sources but are more diluted with picritic or tholeiitic melt fractions formed by progressive melting at shallower levels. Minimum dilution of the incipient low‐degree melts occurs when the melting column is truncated by a thick lithosphere. The mantle source sampled by the Jan Mayen and other alkaline volcanic systems in the NE Atlantic is strongly enriched in high field strength elements like Nb, has a relatively high μ value, and has 207Pb/206Pb ratios below and 208Pb/206Pb ratios above the Northern Hemisphere Reference Line. Such a Pb isotopic composition fits well with an old mantle domain with a time‐integrated low μ value, enriched by a Paleozoic high μ (HIMU) component. The young HIMU‐component in the NE Atlantic upper mantle could represent recycled oceanic crust entrained in the ancestral Iceland plume and distributed laterally by the ancestral plume head.

Some recent Russian studies of ignimbritic rocks

Abstract Summarised translations of recent papers by the Russian geologists Vlodavetz, Petrov, Maleyev, Piip, Aprelkov, and Bykovskaya and Rottman are presented and discussed. Current classifications of volcanic rocks in the U.S.S.R. include a group of rocks, called “tufolavas”, not generally recognised elsewhere. They are reported to be like ash-flow tuffs or ignimbrites, but are characterised by lens-like inclusions and have a “fluidal” or lava-like matrix. Their distinction from welded tuffs deposited at high temperature is not clear in the literature. In the U.S.S.R. ignimbrites and tufolavas have been studied in detail in the Quaternary volcanics of Armenia and Kamchatka. Other, older deposits have been found in volcanic provinces in eastern Siberia, in the Urals, and in the high mountain ranges south of Kazakhstan. Chemically, they range mainly between rhyolite and dacite. Most writers ascribe their origin to some degree of magmatic differentiation. Phenocryst modes differ in the different volcanic ...