Regional igneous rocks

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热心网友 时间：2023-10-19 15:09

In spite of intensive magmatism occurred in the Qiangtang terrane of the Yulong porphyry copper ore-belt by Cenozoic ago（Fig.2），these igneous rocks are not emphases in this book.Here we describe main features of Cenozoic magmatism occurred in the Yulong porphyry copper ore-belt.

1.3.1 Intrusive rocks

1.3.1.1 Geology

A series of Cenozoic porphyries occur in the eastern Qiangtang terrane，which host a number of porphyry copper deposits in the Yulong porphyry copper ore-belt（Fig.2）.From north to south，these porphyries include Xiario，Hengxingcuo，Yulong，Zanaga，Mangzong，Duoxiasongo，Malasongo，Gegongnong，Seli and Mamupu（Fig.2），in which the Yulong porphyry hosts super-giant porphyry copper deposit，the Duoxiasongo and Malasongo porphyries host giant porphyry copper deposits，and the Zanaga and Mangzong porphyries host middle-scale porphyry copper deposits.All the ore-bearing porphyries are the stocks emplaced at hypabyssal to ultra-hypabyssal level.They are composed mainly of monzogranite-porphyry with an outcrop area less than 1 km².These ore-bearing porphyries intruded into the Late Triassic carbonates and clastic rocks in the core of anticlines.Their ages are between 41 and 34 Ma（Table 1）.

Table 1 A compilation of available isotope ages for the ore-bearing porphyries and Cenozoic volcanic rocks in the Yulong porphyry copper ore-belt

1.3.1.2 Petrography

Rock types of ore-bearing porphyries include monzogranite-porphyry，syenogranite-porphyry，alkali-feldspar granite-porphyry，quartz alkali-feldspar syenite-porphyry and quartz monzoniteporphyry（Fig.3）.The rocks show a typical porphyritic texture.The phenocrysts include plagioclase，amphibole，biotite，alkali feldspar，quartz and minor pyroxene.The matrix consists of alkali feldspar，plagioclase，quartz，amphibole and biotite，with a fine-grained granitic texture.Plagioclase phenocryst shows typical normal zoning from andesine in the core to oligoclase at the rim.Plagioclase in the matrix belongs to albite.Biotite is enriched in MgO，belonging to ironphlogopite and eastonite.Some quartz phenocryst has a resorption texture and the pyroxene phenocryst shows partial melting texture，which implies that they probably represent the early-crystallized minerals.

Fig.3 Q′-Anor classification diagram（Streckeisen and Le Maitre，1979）for porphyries in the Yulong porphyry copper ore-belt

The Yulong porphyries are from this study and others are from Ma（1990）and Tang and Luo（1995）

1.3.1.3 Geochemistry

The major element contents of ore-bearing porphyries in the Yulong porphyry copper ore-belt are listed in Table 2.These ore-bearing porphyries have SiO₂contents of 64.3% to 71.2% and thus are acid rocks.The rocks also have high Al₂O₃contents（13.8%～16.4%）with alumina saturation index ASI（＝molar Al₂O₃/（CaO＋Na₂O＋K₂O））values of 0.82～1.55.The first notable geochemical feature of ore-bearing porphyries is that they show shoshonitic affinities.The rocks are enriched in alkalis with K₂O＋Na₂O contents of 7.5%～11.0%（most＞8.0%）.The majority of samples are plotted in the alkaline field（Figs.4a，5）.The rocks also have high K₂O contents（Fig.4b）with high K₂O/Na₂O ratios（Fig.4c）.The majority of samples are plotted in the field of shoshonitic series.High Mg＃（＝Mg/（Mg＋Fe_T））（0.35～0.52）is another notable geochemical feature of ore-bearing porphyries.They are obviously higher than those of pure crustal partial melts（Fig.6）.

The trace element contents of ore-bearing porphyries in the Yulong porphyry copper ore-belt are listed in Table 3.These ore-bearing porphyries are enriched in rare earth elements（REE），especially in light REE（LREE）.They have REE contents of 201×10^-6～481×10^-6with La/Yb ratios of 28～71.Their chondrite-normalized REE patterns show weakly negative or positive Eu anomalies（Fig.7）.The rocks are also enriched in large ion lithophile elements（LILE）（e.g.Rb＝197×10^-6～305×10^-6，Sr＝294×10^-6～2327×10^-6and Ba＝591×10^-6～2271×10^-6）but depleted in high field strength elements（HFSE）（e.g.Nb ＝6×10^-6～16×10^-6，Y＝12×10^-6～21×10^-6）with Sr/Y ratios of 20～111.They show marked negative Nb anomalies in the primitive mantle-normalized trace element patterns（Fig.8）.

Table 2 The major element contents（%）of the ore-bearing porphyries in the Yulong porphyry copper ore-belt

The Yulong porphyries are from this study and others are from Ma（1990）and Tang and Luo（1995）.

The ore-bearing porphyries in the Yulong porphyry copper ore-belt have very similar Sr-Nd i-sotopic compositions（Table 4）.Their ε_Nd（0）values are between -2.0 and -4.1，which are similar to those of the Nangqen volcanic rocks（-0.6 to -3.2）.However，the⁸⁷Sr/⁸⁶Sr ratios（0.7055～0.7073）of the ore-bearing porphyries are slightly higher that those of the Nan-gqen volcanic rocks（0.7050～0.7061）.These Sr-Nd isotopic compositions are distinct from those of the Yarlung-Zangbo MORB and adakites related to slab melting.The Pb isotopic composi-tions of the ore-bearing porphyries are unusually radiogenic.Their²⁰⁶Pb/²⁰⁴Pb，²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios are of 18.71～18.88，15.63～15.67 and 38.89～39.00，respectively（Ta-ble 4），plotting well above the Northern Hemisphere Reference Line（NHRL；Hart，1984）in conventional Pb isotope diagrams.The ore-bearing porphyries show similar²⁰⁶Pb/²⁰⁴Pb ratios to the Nangqen volcanic rocks（18.62～18.97；Deng et al.，2001），but has higher²⁰⁷Pb/²⁰⁴Pb rati-os than the Nangqen volcanic rocks（15.51～15.64；Deng et al.，2001）.The Pb isotope sig-natures of the ore-bearing porphyries and Nangqen volcanic rocks also distinguish those of the Yarlung-Zangbo MORB and adakites related to slab melting.

Fig.4（K₂O＋Na₂O）vs.SiO₂diagram（Le Bas et al.，1986）（a），K₂O vs.SiO₂diagram（b），and K₂O vs.Na₂O diagram（c）for the ore-bearing porphyries in the Yulong porphyry copper ore-belt

The Nangqen volcanic rocks（Sun et al.，1999）are also plotted for comparison.The Yulong porphyries are from this study and others are from Ma（1990）and Tang and Luo（1995）

Fig.5 w（AR）vs.w（SiO₂）（Wright，1969），where AR（Alkalinity Ratio）＝w［Al₂O₃＋CaO＋（Na₂O＋K₂O）］/w［Al₂O₃＋CaO-（Na₂O＋K₂O）］（%）

Symbols and data sources as in Fig.4

Fig.6 Mg＃［＝Mg/（Mg＋Fe_T）］vs.w（SiO₂）diagram

Also shown are the fields of pure crustal partial melt（Rapp and Watson，1995）and worldwide adakite（Xiong et al.，2006 and references therein）.Symbols and data sources as in Fig.4

1.3.2 Volcanic rocks

The Cenozoic volcanic rocks are typically erupted in the strike-slip pull-apart basins such as Gonjo，Nangqen and Lawu（Fig.2）.The volcanism took place mainly in the Eocene（42～34 Ma），which is consistent with the age of emplacement of the ore-bearing porphyries.In the following section，we will expatiate the geological and geochemical features of the Nangqen volcanic rocks that have been investigated in detail by Sun et al.（1999）and Deng et al.（2001）.

Table 3 The trace element contents（×10^-6）of the ore-bearing porphyries in the Yulong porphyry copper ore-belt

The Yulong porphyries are from this study and others are from Ma（1990）and Tang and Luo（1995）.

Fig.7 Chondrite-normalized（Boynton，1984）REE patterns

The Yulong porphyries are from this study and others are from Ma（1990）

Fig.8 Primitive mantle-normalized（McDonough and Sun，1985）trace element patterns

The Yulong porphyries are from this study and others are from Ma（1990）

Table 4 The Sr-Nd-Pb isotopic compositions of the ore-bearing porphyries in the Yulong porphyry copper ore-belt

The Yulong porphyries are from this study and others are from Zhang et al.（1998）.

1.3.2.1 Geology

The distribution of volcanic rocks is consistent with main tectonics in the basin.The volcanism is main overflow of fissure type，and eruption of center type takes second place.The eruption took place in the early stage，and the overflow took place in the later stage.The volcanism includes two cycles.The early cycle formed pyroclastic rocks，which are interlayers of redbeds in the basin.The late cycle formed lavas，subvolcanic rocks and lamprophyre dikes.Nine ages of the Cenozoic volcanic rocks from the basin by K-Ar isotope-dilution technique have been reported by Deng et al.（1999）.The apparent ages of whole rock and separated biotite range from 32.0 to 36.5 Ma（Table 1）.K（%）-⁴⁰Ar_radcorrelation shows a good（r ＝0.99）isochronal line that it passes through the origin of coordinates.An average age of（34.3 ± 0.4）Ma can be obtained.

1.3.2.2 Petrography

The lavas consist mainly of latite and trachyte，with minor shoshonite and tephrite（Fig.4a）.According to Sun et al.（1999），these rocks are greyish green，greyish white and greyish purple.They show a typical porphyritic texture.The phenocrysts（mainly 0.5～5 mm；10% to 30%）include pyroxene and biotite，with minor amphibole，olivine，plagioclase and apatite.The matrix has a trachytic texture，with the mineral components similar to those of phenocrysts.The rocks underwent variable alteration of carbonation and chloritization.

Subvolcanic rocks are mainly syenite-porphyry and monzonite porphyry.They show a porphyritic texture with the phenocrysts of biotite，orthoclase，pyroxene and plagioclase.The matrix has a fine-grained texture.

The lamprophyre dikes include minette and Spessartite.The rocks show a typical lamprophyric texture.Mafic minerals（30% to 70%）mainly include biotite，pyroxene，amphibole and olivine.

1.3.2.3 Geochemistry

The Nangqen volcanic rocks show a wide range of SiO₂contents（43.1%～66.2%）（Table 5）.Their Al₂O₃contents are between 12.4% and 17.2%.The rocks are metaluminous with ASI values of 0.34～0.91.They show shoshonitic affinities.In spite of ultrabasic-basic or intermediate or acid rocks，they are enriched in alkalis（Figs.4a，5）with high contents of K₂O（Fig.4b）and high K₂O/Na₂O ratios（Fig.4c）.The rocks have Mg＃ of 0.43～0.71（Table 5），which are obviously higher than those of pure crustal partial melts（Fig.6）.In term of trace element contents（Table 6），the Nangqen volcanic rocks also show shoshonitic affinities.They are enriched in REE，especially in LREE.They have REE contents of 240×10^-6～1260×10^-6and show a decrease trend from ultrabasic-basic through intermediate to acid rocks（Fig.9）.Their La/Yb ratios are between 41 and 102.Their chondrite-normalized REE patterns lack negative Eu anomalies（Fig.9）.The rocks are also enriched in LILE（e.g.Rb＝60×10^-6～186×10^-6，Sr＝641×10^-6～6600×10^-6and Ba＝1024×10^-6～4269×10^-6）but relatively depleted in HFSE（e.g.Nb＝9.7×10^-6～33×10^-6，Ta ＝0.74×10^-6～1.98×10^-6）.They show marked negative Ta-Nb，Zr-Hf and Ti anomalies in the primitive mantle-normalized trace element patterns（Fig.10）.

In spite of ultrabasic-basic or intermediate or acid compositions，the Nangqen volcanic rocks have very similar Sr-Nd-Pb isotopic compositions（Table 7）.Their⁸⁷Sr/⁸⁶Sr ratios are 0.7050～0.7056 for ultrabasic-basic rocks，0.7052～0.7061 for intermediate rocks and 0.7052 for acid rocks.Their ε_Nd（0）values are -0.6 to -2.6 for ultrabasic-basic rocks，-0.6 to -2.2 for intermediate rocks and -1.3 to -3.2 for acid rocks.The Pb isotopic compositions of the Nangqen volcanic rocks are unusually radiogenic，with the data plotted well above the Northern Hemisphere Reference Line（NHRL；Hart，1984）in conventional Pb isotope diagrams.Their²⁰⁶Pb/204 Pb，²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios are of 18.62～18.78，15.52～15.61 and 38.43～38.73，respectively，for ultrabasic-basic rocks，of 18.63～18.97，15.51～15.64 and 38.51～39.00，respectively，for intermediate rocks，and of 18.86～18.87，15.58～15.60 and 38.75～38.81，respectively，for acid rocks.

Table 5 The major element conterts（w_B/%）of the Nangqen volcanic rocks

Rock type:LAP=Lamprophyre,PT=Phonotephrite,TP=Tephriphonolite,SH=Shoshonite,LA=Latite,TR=Trachyte,SP=Syenite-porphyry,TD=Trachydacite.The original data are from Sun et al.（1999）.

Table 6 The trace element contents（×10^-6）of the Nangqen volcanic rocks

continued

Rock types as in Table 5.

The original data are from Deng et al.（2001）.

Fig.9 Chondrite-normalized（Boynton，1984）REE patterns of the Nangqen volcanic rocks

The original data are from Deng et al.（2001）

Fig.10 Primitive mantle-normalized（McDonough and Sun，1985）

trace element patterns of the Nangqen volcanic rocks

The original data are from Deng et al.（2001）

1.3.2.4 Petrogenesis

Based on the petrology，mineralogy and geochemistry，Deng et al.（2001）conclude that the relations between ultrabasic-basic，intermediate and acid rocks are neither differentiation nor evolution.Instead，their geochemical variability is mainly attributed to inhomogeneity of source region and to different degrees of partial melting.The source regions are the enriched metasomatic mantle that might have undergone a multiple mixing process.The enriched components are mainly crustal rocks that were most likely recycled into the mantle wedge ring the ancient subction.The generation of the Nangqen volcanic rocks is mainly controlled by the large-scale strikeslip fault system，which not only inces the upwelling of asthenosphere and thus becomes heat engine of volcanic rocks，but also provides conits for the melt ascent.

Table 7 The Sr-Nd-Pb isotopic compositions of the Nangqen volcanic rocks

The original data are from Deng et al.（2001）.

1.3.3 Mantle xenoliths hosted by the Ailaoshan-Jinshajiang Cenozoic alkalirich porphyries

Although it has not been reported that the porphyries in the Yulong porphyry copper ore-belt contain the mantle xenoliths，some porphyry bodies within the Ailaoshan-Jinshajiang Cenozoic alkali-rich porphyry belt to the southeast of Yulong contain a series of upper mantle xenoliths（Liu et al.，2002，2003；Zhao et al.，2003）.These mantle xenoliths，mainly within the Liuhe aegirine-augite syenite-porphyry，western Yunnan，include garnet diopsidite，phlogopite-garnet diopsidite and garnet-phlogopite diopsidite.The rocks consist mainly of clinopyroxene，garnet and mica with a medium-to coarse-grained texture.Accessory minerals include rutile and magnetite.

Pyroxene in the mantle xenoliths has high CaO and MgO and low Al₂O₃contents（Table 8），belonging to Ca-rich clinopyroxene（mainly diopside）.Garnet in the mantle xenoliths has miner al end-member of almandine（40%～50%），pyrope（22%～36%）and essonite（15%～22%）.Mica is also characterized by high MgO contents（Table 9），belonging to phlogopite and iron-phlogopite.

Table 8 Electron microprobe analyses of pyroxene in the mantle xenoliths（w_B/%）

Mg＃＝Mg/（Mg＋Fe_T）；the order 1 to 7 are from Liu et al.（2002）；and the order 8 is from Zhao et al.（2003）.

Table 9 Electron microprobe analyses of mica in the mantle xenoliths（w_B/%）

Mg＃＝Mg/（Mg＋Fe_T）；the original data are from Liu et al.（2002）.

The chemical compositions of mantle xenoliths are listed in Table 10，which indicate that these mantle xenoliths are enriched in TiO₂，Fe₂O₃，CaO，Al₂O₃，Na₂O，K₂O，CO₂and P₂O₅，and depleted in MgO compared to spinel lherzolite.In terms of trace element contents，the mantle xenoliths are enriched in LREE and HFSE，and depleted in Th and Sr（Fig.11a，b）.

Table 10 The major（w_B/%）and trace（×10^-6）element contents of the mantle xenoliths

GD＝garnet diopsidite，PGD＝phlogopite-garnet diopsidite，GPD＝garnet-phlogopite diopsidite.

The original data are from Liu et al.（2003）.

Fig.11 Chondrite-normalized（Boynton，1984）REE pattern（a）and primitive mantle-normalized（McDonough and Sun，1985）trace element pattern（b）of the mantle xeonliths

The original data are from Liu et al.（2003）