40Ar/39Ar data from the Talkeetna Mountains C-4 Quadrangle and adjoining areas, central Alaska

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Frequently anticipated questions:

What does this data set describe?

Title:
40Ar/39Ar data from the Talkeetna Mountains C-4 Quadrangle and adjoining areas, central Alaska
Abstract:
This report presents 40Ar/39Ar step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) Talkeetna Mountains C-4 Quadrangle and Adjoining Areas geologic mapping project. Hornblende-bearing gabbro returned a Late Triassic crystallization age consistent with regional results for Nikolai Greenstone-related magmatism. We determined Early Jurassic metamorphic ages for hornblende and sericite from two samples of upper greenschist to amphibolite grade metavolcanic rocks. Our results indicate that a granodiorite pluton intruding Wrangellia in the study area has a Middle Jurassic age consistent with the Talkeetna Arc, indicating that the Wrangellia and Peninsular terranes were joined at that time. Cretaceous plutons and porphyry intrusions in the map area can be divided into two types: reduced granitic to felsic porphyry intrusions with latest Cretaceous ages, and an earlier, oxidized type with early Late Cretaceous ages. This observation corresponds to similar patterns in magmatism in the western Alaska Range, and may indicate geological potential for similar style intrusive-related gold, copper, and molybdenum mineralization in the study area. Finally, we obtained Eocene ages for andesite and rhyolite; these ages constrain the timing of displacement on some of the faults in the map area. Analyses were performed by the University of Alaska Fairbanks (UAF) Geochronology Laboratory, and the results were reported by Jeff Benowitz and Paul Layer. Products included in this data release are a summary of sample collection methods, the laboratory report, analytical data tables and associated metadata, and the plots of the 40Ar/39Ar age spectra, Ca/K, and Cl/K ratios.
Supplemental_Information: 
border:    Outline of the study area
arar-summary-data-locations:    Identification numbers, coordinates, sample descriptions, and summary ages of each sample
step-heating-data:    Power output levels and resultant measurements from each sample run
  1. How should this data set be cited?

    Benowitz, J.A., Layer, P.W., Wypych, Alicja, and Twelker, Evan, 2015, 40Ar/39Ar data from the Talkeetna Mountains C-4 Quadrangle and adjoining areas, central Alaska: Raw Data File RDF 2015-10, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 19 p.

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -148.944298
    East_Bounding_Coordinate: -148.435438
    North_Bounding_Coordinate: 62.735032
    South_Bounding_Coordinate: 62.401607

  3. What does it look like?

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 2015
    Ending_Date: 22014
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: report and digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a point data set.

    2. What coordinate system is used to represent geographic features?

      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.00001. Longitudes are given to the nearest 0.00001. Latitude and longitude values are specified in decimal degrees.

      The horizontal datum used is World Geodetic System of 1984.
      The ellipsoid used is WGS 84.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257223563.

  7. How does the data set describe geographic features?

    rdf2015-10-border.shp
    Outline of the study area (Source: Alaska Division of Geological & Geophysical Surveys)

    rdf2015-10-arar-summary-data-locations.csv
    Identification numbers, coordinates, sample descriptions, and summary ages of each sample (Source: Alaska Division of Geological & Geophysical Surveys)

    SAMPLE
    Label assigned in the field to identify the sample. (Source: Alaska Division of Geological & Geophysical Surveys)

    Generic example of a sample identifier: YYAAA9999X: YY=last two digits of year, AAA=geologist's initials (one to three characters), 9999=unique station number, X= optional alpha character which indicates that multiple samples were collected at a given location or that multiple observations were recorded in the project database.

    LAB_SAMPLE_NUMBER
    Label used by the lab to identify the sample. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    The lab sample number is typically identical to the field sample number. However, in some cases, the sample number provided by the lab does differ from the number assigned in the field. We provide both numbers to document instances in which the sample numbers in the lab files differ from the field sample number.

    COLLECTOR
    The geologist who collected the sample. (Source: Alaska Division of Geological & Geophysical Surveys)

    The samples presented in this report were collected by the authors or other DGGS staff.

    LONGITUDE
    Longitude, WGS84 (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:-148.944298
    Maximum:-148.435438
    Units:decimal degrees

    LATITUDE
    Latitude, WGS84 (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:62.401607
    Maximum:62.735032
    Units:decimal degrees

    DATED_MATERIAL
    Indicates the type of material that was used for analysis (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    The type of material that was selected for dating varied according to mineral composition and grainsize of the submitted rock sample. This field specifies whether the age was derived from a mineral separate (generally 1 to 10 individual crystals dated together as a single run), a single crystal, or the whole rock.

    MINERAL
    Indicates the mineral that was used for analysis (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the sample was a whole rock or that the mineral type was not recorded by the lab

    The minerals selected for dating would typically be grains of biotite, hornblende, or muscovite.

    DESCRIP_1
    Description of the sample (Source: Alaska Division of Geological & Geophysical Surveys)

    Characters 1 to 254 of a brief sample or station description.

    DESCRIP_2
    Extension of the sample or station description for descriptions which are greater than 254 characters in length (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NULLBlank (NULL) values indicate that the descriptive text is shorter than 254 characters.

    Characters greater than 254 of the sample or station description.

    DESCRIP_3
    Extension of the sample or station description for descriptions which are greater than 509 characters in length (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NULLBlank (NULL) values indicate that the descriptive text is shorter than 509 characters.

    Characters greater than 509 of the sample or station description.

    INTEGRATED_AGE_MA
    Integrated age and age error in million years (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    The values are a concatenation of the integrated age determination and the calculated error associated with the integrated age determination

    PLATEAU_AGE_MA
    Plateau age and age error in million years (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the PLATEAU_AGE_MA value was not calculated by the lab

    The values are a concatenation of the plateau age determination and the calculated error associated with the plateau age determination

    PLATEAU_REMARKS
    Comments and additional values that pertain to the plateau age determination method or interpretation. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the PLATEAU_REMARKS value was not provided by the lab

    Free text

    W_M_AGE_MA
    Weighted mean age and age error in million years (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the W_M_AGE_MA value was not calculated by the lab

    The values are a concatenation of the weighted mean age determination and the calculated error associated with the weighted mean average age determination

    W_M_AGE_REMARKS
    Comments and additional values that pertain to the weighted mean age determination method or interpretation. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the W_M_AGE_REMARKS value was not provided by the lab

    Free text

    ISOCHRON_AGE
    Isochon age and age error in million years (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the ISOCHRON_AGE value was not provided by the lab

    The values are a concatenation of the isochron age determination and the calculated error associated with the isochron age determination

    ISOCHRON_REMARKS
    Comments and additional values that pertain to the isochron age determination method or interpretation. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the ISOCHRON_REMARKS value was not provided by the lab

    Free text

    AGE_TYPE_PREFERRED
    Indicates AGE_TYPE_PREFERRED age. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    A discussion of the preferred age for each analysis is provided in the accompanying lab report. The possible values are: plateau, isochron, weighted average, or integrated.

    PREFERRED_AGE
    The values are a concatenation of the preferred age determination and the calculated error associated with the preferred age determination (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    The preferred age for each analysis is discussed in detail in the accompanying lab report.

    rdf2015-10-step-heating-data.csv
    Power output levels and resultant measurements from each sample run (Source: Alaska Division of Geological & Geophysical Surveys)

    SAMPLE
    Label assigned in the field to identify the sample. (Source: Alaska Division of Geological & Geophysical Surveys)

    Generic example of a sample identifier: YYAAA9999X: YY=last two digits of year, AAA=geologist's initials (one to three characters), 9999=unique station number, X= optional alpha character which indicates that multiple samples were collected at a given location or that multiple observations were recorded in the project database.

    LAB_SAMPLE_NUMBER
    Label used by the lab to identify the sample. (Source: Alaska Division of Geological & Geophysical Surveys and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    The lab sample number is typically identical to the field sample number. However, in some cases, the sample number provided by the lab does differ from the number assigned in the field. We provide both numbers to document instances in which the sample numbers in the lab files differ from the field sample number.

    Laser_mW
    Laser output level in megawatts; note: sample 14LL220 FS#L1 was heated to fusion, the exact laser output level was not explicitly recorded (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the values in the row were calculated by weighting the individual heating steps by the fraction of 39Ar released.

    Range of values
    Minimum:400
    Maximum:>9001
    Units:megawatts

    Integrated
    We indicate whether the values in the row were derived from a heating step or calculated by weighting the individual heating steps by the fraction of 39Ar released. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NULLThe values in the row were derived from a heating step
    Integratedthe values in this row were calculated by weighting the individual heating steps by the fraction of 39Ar released

    Cum39Ar
    Cumulative ratio of 39Argon released at this step (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    ValueDefinition
    NULLA NULL or blank values indicates that the value was not applicable or not calculated by the lab

    Range of values
    Minimum:0.0008
    Maximum:1

    Me40Ar39Ar
    Measured 40Ar/39Ar ratio for each heating step, corrected for blank and 39Ar decay (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:8.83842
    Maximum:687.05728

    Er40Ar39Ar
    The one sigma error for each 40Ar/39Ar measurement (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.01033
    Maximum:151.04861

    Me37Ar39Ar
    The ratio of measured 37Ar to measured 39Ar for each heating interval for each sample corrected for blank and 37Ar and 39Ar decay (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.0018
    Maximum:35.85506

    Er37Ar39Ar
    The one sigma error for each 37Ar/39Ar measurement (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.00019
    Maximum:4.49223

    Me36Ar39Ar
    Ratio of measured 36Ar to measured 39Ar for each step interval for each sample corrected for blank and 39Ar decay (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:-0.03748
    Maximum:2.07748

    Er36Ar39Ar
    The one sigma error for each 36Ar/39Ar measurement (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.00004
    Maximum:0.5007

    Atm40Ar
    Percent of 40Ar which is atmospheric in composition at each step level (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:-21.9135
    Maximum:100.00597

    ErAtm40Ar
    The one sigma error for the percent of 40Ar which is atmospheric in composition at each step level (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.05812
    Maximum:38.70786

    MeCaK
    Ratio of Calcium to Potassium as determined from 37Ar/39Ar (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.00331
    Maximum:67.49824

    ErCaK
    The one sigma error for each Ca/K determination (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.00034
    Maximum:8.45986

    MeClK
    Ratio of Chlorine to Potassium as determined from 38Ar/39Ar (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.0004
    Maximum:0.3719

    ErClK
    The one sigma error for each Cl/K determination (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.00003
    Maximum:0.0343

    Er40ArRad39ArK
    One sigma error for each 40ArRad/39ArK value (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.0153
    Maximum:67.18624

    AgeMa
    Calculated age for each temperature step for each sample (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:-0.08
    Maximum:556.21
    Units:MA

    ErAgeMa
    One sigma error of the age determination of each heating step (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    Range of values
    Minimum:0.13
    Maximum:296.96
    Units:MA

    WeightedAverageOfJFromStandards
    J value, calculated from the weighted average of standards placed in various positions in the nuclear reactor (Source: Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

    WeightedAverageOfJFromStandards value and error

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    This project is funded through the US Geological Survey's National Cooperative Geologic Mapping Program STATEMAP component (award number G14AC00167) and Governor's Strategic and Critical Minerals Assessment Capital Improvement Project, a component of the ongoing Alaska Airborne Geophysical/Geological Mineral Inventory Program funded by the Alaska State Legislature and managed by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys.

  3. To whom should users address questions about the data?

    Alaska Division of Geological & Geophysical Surveys
    GIS Manager
    3354 College Road
    Fairbanks, AK 99709-3707
    USA

    (907)451-5020 (voice)
    dggsgis@alaska.gov

    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays

Why was the data set created?

The U.S. Geological Survey (USGS) National Cooperative Geologic Mapping Program partially funded work by DGGS to map approximately 450 mi2 of the Talkeetna Mountains region of central Alaska at 1:50,000 scale. This area contains significant exposures of Late Triassic mafic volcanics and gabbro sills that have been the focus of region-wide exploration for the Strategic and Critical platinum-group elements (PGEs). The area also exposes numerous inactive and possibly active faults that project through the area of the proposed Susitna-Watana hydroelectric dam and reservoir. Because this area was poorly mapped at a reconnaissance scale of 1:250,000, a team of eight DGGS geologists spent 42 days in the field to produce a new geologic map and a greatly improved understanding of the geology, structural history, and mineral potential of the area.

How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: 2014 (process 1 of 2)
    Fieldwork - Fresh, unweathered samples from surficial outcrops were collected by DGGS field geologists; samples were selected based on presence of sufficiently large crystals and/or fresh glassy matrix. Sample location coordinates (in WGS84 datum) were obtained using handheld Trimble Juno T5 GPS units, with typical reported accuracy of about 10 m.

    Date: 2014 (process 2 of 2)
    Sample preparation and analysis - Before processing, samples were examined under binocular microscope, or thin sections were prepared and scrutinized to eliminate the possibility of analyzing altered mineral phases. Rock samples that we found to be suitable for dating by 40Ar/39Ar geochronology analysis were submitted to the Geochronology Laboratory at the University of Alaska Fairbanks (UAF) where they were dated according to the protocols outlined in the accompanying report.

  3. What similar or related data should the user be aware of?

    Benowitz, J.A., Layer, P.W., and Twelker, Evan, 2014, 40Ar/39Ar data from rocks collected in 2013 in the Wrangellia mineral assessment area, Gulkana, Healy, Mount Hayes, and Talkeetna Mountains quadrangles, Alaska: Raw Data File RDF 2014-18, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 10 p
    Twelker, Evan, Bachmann, E.N., Freeman, L.K., Newberry, R.J., Reioux, D.A., Sicard, K.R., Tuzzolino, A.L., Wright, T.C., and Wypych, Alicja, 2014, Major-oxide, minor-oxide, and trace-element geochemical data from rocks and stream sediments in the Wrangellia mineral assessment area, Gulkana, Healy, Mount Hayes, and Talkeetna Mountains quadrangles, Alaska: Raw Data File RDF 2014-3, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 6 p
    Werdon, M.B., Azain, J.S., and Granitto, Matthew, 2014, Reanalysis of historical U.S. Geological Survey sediment samples for geochemical data from the western part of the Wrangellia terrane, Anchorage, Gulkana, Healy, Mt. Hayes, Nabesna, and Talkeetna Mountains quadrangles, Alaska: Raw Data File RDF 2014-5, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 6 p
    Wypych, Alicja, Twelker, Evan, Freeman, L.K., Lande, Lauren, Newberry, R.J., Reioux, D.A., and Sicard, K.R., 2014, Major-oxide and trace-element geochemical data from rocks collected in 2014 in the Wrangellia mineral assessment area, Talkeetna Mountains C-4, C-3, and B-4 quadrangles, Alaska: Raw Data File RDF 2014-22, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 4 p

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    Coordinates of sample locations were determined by the geologists in the field using handheld GPS units. Sample numbers were recorded in the field on sample bags and on field note sheets. The sample location table was cross checked for consistency against the sample numbers recorded on the bags. Laboratory sample numbers were generated by the UAF geochronology, in some cases the lab numbers differ from those assigned by DGGS geologists. We provide both the lab and the DGGS sample numbers in the data tables. See text report for additional discussion of laboratory procedures. Note: sample 14LL220 FS#L1 was heated to fusion, the exact laser output level was not explicitly recorded. Lab results and interpretations were examined by several geologists to ensure that they are reasonable given the geologic context of the sample.

  2. How accurate are the geographic locations?

    Sample location coordinates (in WGS84 datum) were obtained using handheld Trimble Juno T5 GPS units, with typical reported accuracy of about 10 m.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    Users should note that the interpretive summaries that are provided in the text report have been lightly edited for clarity but are generally provided as received from the lab.

  5. How consistent are the relationships among the observations, including topology?

    not applicable

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints:
This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

  1. Who distributes the data set? (Distributor 1 of 1)

    Alaska Division of Geological & Geophysical Surveys
    3354 College Road
    Fairbanks, AK 99709-3707
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov

    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
    Contact_Instructions:
    Please view our website (<http://www.dggs.alaska.gov>) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
  2. What's the catalog number I need to order this data set?

    RDF 2015-10

  3. What legal disclaimers am I supposed to read?

    The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 16-Jun-2015
Metadata author:
Alaska Division of Geological & Geophysical Surveys
Metadata Manager
3354 College Road
Fairbanks, AK 99709-3707
USA

(907)451-5020 (voice)

Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:

Generated by mp version 2.9.21 on Tue Jun 16 11:25:46 2015