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Analytical method
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Geochronology
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Zircons separated from plutonic granitoids were dated using LA-ICPMS following the method described by Iizuka and
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Hirata (2004). Besides the geochronological information, we aimed at identifying inherited zircons to get a qualitative
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impression of the role of assimilation in the granitoid formation. Therefore, we used beside the international zircon
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standard 91500 also as “in house” standard, zircons from sample 01B22 previously dated by conventional TIMS U-Pb
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zircon dating and shown to have inherited zircons (Heuberger, et al. 2007) to evaluate the precision of the applied
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method.
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Zircons were obtained after crushing and sieving using magnetic and heavy liquid separation methods. Where possible
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70-100, hand picked, long prismatic, inclusion free, idiomorphic zircons where mounted in epoxy resin, polished and
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analysed with an inductively coupled plasma mass spectrometer (ICP-MS) combined with a laser-ablation system (LA)
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at the Tokyo Institute of Technology, Japan. We used a ThermoElectron VG PlasmaQuad 2 quadrupole ICPMS
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equipped with the S-option interface (Hirata and Nesbitt 1995) coupled with a GeoLas 200CQ laser ablation system
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(MicroLas, Göttingen, Germany). This system utilizes Lambda Physik (Göttingen, Germany) COMPex 102 193 nm
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ArFexcimer laser. The repetition rate of the laser system was 6 Hz, the beam diameter was 16 µm, and the laser output
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energy was 140mJ. NIST 610 SRM was used as standard material.
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The common Pb content of the analysed zircons was generally negligible and within 2σ error of the measured 204Hg gas
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blank. However, a single sample (BO-02-13) had few analyses with significant amounts of
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using common lead composition according to the model of Stacy and Kramers (1975). This correction had negligible
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influence on the calculated age.
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A general problem of dating young zircons is the low concentration of 207Pb resulting in poor counting statistics. When
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concentration approaches the limit of detection uncertainty of up to a few of tens of percent dominate analytical
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uncertainty. As the 207Pb count rate is on the same order of magnitude as the 204Hg gasblank count rates, we used the 2σ
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of the later to approximate the error on
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considered meaningless and in part are geologically unrealistic (i.e. future ages Table A1). All ages presented in this
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study are however, more precise
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youngest population of zircons yields an age of 44.8 ± 5.6 (2σ) Ma consistent with the conventional U-Pb TIMS zircon
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age of 49.80 ± 0.15 Ma (Heuberger, Schaltegger, Burg, Villa, Frank, Dawood, Hussain and Zanchi 2007). Beside older
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population also documented by U-Pb TIMS data additional younger zircon population have been found by LA-ICPMS.
207/206Pb
206Pb/238U
to be 20 %. Due to analytical difficulties in
204Pb
207Pb
which was corrected
all
207/206Pb
ages are
ages. The U-Pb ages of standard 01B22 zircons are well reproduced. The
1
It has been demonstrated (e.g. Hirata and Nesbitt 1995) that matrix-matched calibration is required in order to achieve
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accurate LA-ICP-MS U-Pb ages for some analytical setups. However, recent studies utilizing an examiner laser with a
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high energy density demonstrate that the analytical uncertainty due to matrix effects between the NIST 610 glass
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standards and zircon crystal are negligible at the 2σ precision level (Iizuka and Hirata 2004). This observation is
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consistent with the result of the “in house” standard. The TIMS U-Pb age of sample 01B22 is well reproduced even at
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the 95% C.I. (Table A1) indicating that the 2σ probably overestimate the analytical uncertainty. Based on these results,
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we are confident that LA-ICP-MS analyses on young, homogeneous “near” concordant zircons result in ages reflecting
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the time of intrusion within 2σ analytical uncertainties. Averaged ages given in Table 1 are quoted with a 2σ error.
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MSWD values have been calculated using Ken Ludwig’s isoplot program V (Ludwig 2000).
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Whole rock geochemistry
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Whole rock major element compositions were determined either by Acta labs (Canada) by ICP-OES and ICP-MS
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(www.actalabs.com) or using a Rigaku RINT 2000 powder X-ray diffractometer at the Tokyo Institute of Technology,
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Japan. For XRF analysis, samples were ground to fine powder in an agate ball mill, taking care not to include any
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veined or weathered material, and all powders were oven-dried overnight at 105 °C. Standard glass pills have been
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prepared using lithium tetraborate fusion method. The trace element concentration of the samples analysed by XRF
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were determined on the XRF glass pills using XRF major element data (Al2O3) as an internal standard. For NIST 610
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used as standard material we used the preference values of Spandler et al (in prep)
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Trace element data have been determined on the Li2B4O7-fused XRF pellets directly using LA-ICP-MS at the
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University of Bern. The setup consists of a GeoLas Pro 193 nm ArF excimer laser system with homogenized energy
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density across the ablation pit (Microlas), linked with an Elan DRCe ICP quadrupole MS (Perkin Elmer). The XRF
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pellets were broken, and analyses were performed directly on freshly broken surfaces far away from the original
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surfaces of the pellets. Samples were loaded along with the SRM 610 glass standard from NIST in a 20 cm3 ablation
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cell and the laser-ablation aerosol was carried to the ICP-QMS by a mixed He-Ar carrier gas. The analytical set-up was
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tuned for optimum performance across the entire mass range. Daily optimization of the analytical conditions were done
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to satisfy a ThO production rate of below 0.2% (i.e., Th/ThO intensity ratio < 0.002) and a Th/U sensitivity ratio of 1 as
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determined on the SRM610 glass standard. Two analyses on the external standard at the beginning and the end of each
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set bracketed up to 16 analyses of unknowns. A minimum of 3 shots per pellet were done to control for homogeneity of
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the analyzed pellet. We used a 100 µm pit to lower the LOD. The certified glass standard SRM 610 was used as an
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external standard to calibrate analyte sensitivities, and bracketing standardization provided a linear drift correction.
1
Data reduction of LA-ICP-MS analyses followed procedures described in Longerich et al. (1996). We used CaO as the
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internal standard element to calculate trace element concentrations. Limits of detection (LOD) for each signal interval
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were calculated for each element, for individual analyses, as three times the standard deviation of the gas background
4
signal divided by the element sensitivity (Longerich et al., 1996).
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Sample description
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A collateral objective of this study was to elucidate the relationship between stage 1 and stage 2 plutons in the Dir area
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following the earlier work of Sullivan et al. (1992, e.g. their Fig 2 and 3). To begin this work, we followed the
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description of Petterson and Windley (1985) and grouped the samples to be dated according to the presence (stage 1) or
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absence (stage 2) of deformation fabric. Sampling resulted in stage 1 plutons (BR-02-19) from the south of the Dir
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group and stage 2 plutons (DR-02-18, BO-02-13; MN-02-04, MR-02-03 and Rb-02-16) from the north (Figure 2). In
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addition, we sampled a strongly deformed quartz-rich tonalite from the eastern part of the southern Kohistan Batholith
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(C-01-77, Fig1), which has all the characteristics of stage 1 plutons (Table 1). Detailed sample descriptions are given
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bellow:
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C-01-77 (35°33´003´´N; 74°09´129´´E)
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A strongly foliated tonalite from the southern limit of the Kohistan arc in the eastern part of Kohistan crops out near the
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town of Chilas (Khiner valley). This tonalite contains numerous volcanic xenoliths and is crosscut by fine grained
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amphibolitic dykes. The magmatic mineral assemblage is well preserved: plagioclase, quartz, K-feldspar, biotite,
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allanite, epidote, FeTi-oxide, zircon and apatite. The shape of the grain aggregates is seriate – interlobate. A strong
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foliation is defined by the orientation of biotite and quartz ribbons. Quartz crystals within the ribbons have mm-scale
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grain size and generally show straight extinction whereas large (cm-scale) phenocrystic quartz and plagioclase crystals
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show undulose extinction. Subgrain boundaries and deformation bands are present within quartz crystals. Plagioclase
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and quartz crystals have interlobate grain boundaries indicating dynamic recrystallization. Tapered twins within
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plagioclase and deformation bands in quartz crystals are frequent. In summary the textures of C-01-77 indicate strong,
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high temperature deformation of the magmatic protolith and, accordingly, the sample displays the deformational
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features of stage 1 plutons.
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MN-02-04 (35°34´646´´N; 72°45´599´´E)
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MN-02-04 is a coarse grained (> 1 cm) unfoliated rock with abundant volcanic xenoliths demonstrating the intrusive
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relationship with the surrounding Dir-Utror volcanic units. The magmatic mineralogy is composed of hypidiomorphic
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plagioclase, hornblende, quartz, biotite, K-feldspar, FeTi-oxide, zircon and apatite. A greenshist facies metamorphic
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overprint is indicated by alteration of biotite into chlorite and titanite. Plagioclase is altered to clinozoisite and K-
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feldspar into phengite. Stilpnomelane is present as a metamorphic phase. Quartz crystals sometimes display patchy
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undulose extinction. Grain boundaries are generally straight; however, a few are interlobate, indicating weak grain
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boundary migration. Accordingly sample MN-04-02 is a weakly to undeformed rock which would classify as a stage 2
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pluton.
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MR-02-03 (35°33´205´´N; 72°33´451´´E)
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Interpretation of Landsat ETM + pictures and field observation indicate that MR-02-3 and Mn-02-04 are from the same
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coarse grained hornblende-bearing granite body. MR-02-3 was sampled in the UshuValley, approximately 10 km to the
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east of MN-02-04.
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MR-02-03 is a fine-grained tonalite that intrudes the Dir-Utror volcanic units. It is locally transected by high strain
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zones. Towards the intrusive contact, the tonalite contains abundant xenoliths of volcanic material. In the contact zone
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apophyses are common in the volcanic country rocks. The magmatic mineralogy is composed of plagioclase, quartz,
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biotite, minor K-feldspar, FeTi-oxide, titanite, zircon and apatite. This magmatic mineral assemblage is overprinted by
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greenschist facies metamorphism indicated by the same mineral reaction as described for MN-02-04.A strong foliation
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is defined by the alignment of biotite and ribbons of small (mm-big) quartz grains anastomosing around larger feldspar
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crystals (mantle structure). Plagioclase shows frequent tapered twins. Feldspar grains are fractured with bookshelf
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rotation of neighbouring framents. Grain boundaries are interlobate to seriate and transition from high angle grain
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boundaries to subgrain boundaries exists. The fabric demonstrates low temperature (greenschist facies) deformation of
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the magmatic protholith. The fabric is in accordance with stage 1 features, in contrast with the supposed stage 2
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inferred from the fabric of sample MN-02-04. However, we attribute the low temperature deformation of MN-02-03 to
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local thrusting along the nearby Dir Fault.
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BO-02-13 (35°22´579´´N; 72°03´257´´E)
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This medium to coarse grained quartz-diorite with volcano-sedimentary xenoliths is crosscut by tonalite dykes. The
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magmatic paragenesis of the quartz diorite is composed of hypidiomorphic concentrically zoned plagioclase,
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hornblende, quartz, biotite, FeTi-oxide, titanite, zircon and apatite. Magmatic hornblende is zoned with actinolite rims
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indicating greenschist facies overprint. The rock is macroscopically unfoliated. However, in thin section cm-scale
1
plagioclase phenocrysts are separated by ca. 0.5 cm wide anastomosing quartz ribbons composed of mm-scale quartz
2
crystals. The quartz grains within the ribbons display undulose extinction and interlobate grain boundaries indicating
3
dynamic recrystallization. Accordingly the deformed rock would classify as stage 1.
4
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RB-02-16 (35°33´712´´N; 72°12´666´´E)
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This whitish coarse grained (~2 cm) biotite-titanite granite has a magmatic mineral assemblage that comprises quartz,
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K-feldspar, plagioclase, biotite, allanite, FeTi-oxide, titanite, zircon and apatite. Allanite is rimed by metamorphic
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clinozoisite. Plagioclase displays concentric zoning partly overprinted by tapered twins. K-Feldspar phenocrysts show
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patchy undulose extinction associated with brittle fracture zones. Grain boundaries are interlobate; however, straight
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grain contacts exist. Quartz display undulose and straight extinction. Besides the undulose K-feldspar and quartz grains
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the rock is undeformed and therefore is classified as stage 2.
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DR-02-18 (35°14´497´´N; 71°51´641´´E)
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A coarse-grained diorite has spread numerous apophyses into the surrounding Dir-Utror volcanic rocks. The magmatic
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mineralogy is composed of plagioclase, clinopyroxene, orthopyroxene, quartz, biotite, FeTi-oxide. Pyroxenes are partly
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rimed by hornblende. Myrmekite is common. A weak foliation is defined by the preferred orientation of biotite. Quartz
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crystals sometimes show undulose extinction. Grain boundary migration is demonstrated by the bulging of quartz
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grains into plagioclase. The weak deformation features classify the rock as stage 2.
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BR-02-19 (35°07´687´´N; 72°56´229´´E)
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Like DR-02-18, this coarse grained (<1cm) quartz-diorite has many apophyses indicating intrusion into the volcanic
22
units. The granodiorite is in turn intruded by leucocratic pegmatites. The magmatic mineralogy of the dated
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granodiorite is composed of quartz, plagioclase, K-feldspar, biotite, FeTi-oxide, zircon and apatite. The alignment of
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biotite crystals defines the foliation. Quartz has generally undulose extinction. Deformation bands in quartz are
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common and interlobate to seriate grain boundaries indicate dynamic recrystallization. Tapered twins in plagioclase are
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common. Transition between high angle grain boundaries and subgrain boundaries in quartz is common. The fabric
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indicates high-temperature deformation and therefore the sample is classified stage 1.
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Table A1: Results U-Pb LA-ICPMS zircon analysis compared to conventional U-Pb TIMS ages for zircons of known
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ages.
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